diff --git a/addons/mail/models/ir_model.py b/addons/mail/models/ir_model.py
index faaea2b3d73..22363f555ca 100644
--- a/addons/mail/models/ir_model.py
+++ b/addons/mail/models/ir_model.py
@@ -106,25 +106,33 @@ class IrModel(models.Model):
         for model_name in model_names_to_fetch:
             model = self.env[model_name]
             # get fields, relational fields are kept only if the related model is in model_names_to_fetch
-            fields_by_fname = {
-                fname: field
-                for fname, field in model.fields_get(
+            fields_data_by_fname = {
+                fname: field_data
+                for fname, field_data in model.fields_get(
                     attributes=['name', 'type', 'relation', 'required', 'readonly', 'selection', 'string']
                 ).items()
-                if not field.get('relation') or field['relation'] in model_names_to_fetch
+                if not field_data.get('relation') or field_data['relation'] in model_names_to_fetch
             }
             # exclude date/datetime and binary default values: dates will always be wrong since they are dynamic
             # and binary values are not needed for now
             default_values_by_fname = model.default_get([
-                fname for fname, field in fields_by_fname.items()
-                if field['type'] not in ['binary', 'date', 'datetime']
+                fname for fname, field_data in fields_data_by_fname.items()
+                if field_data['type'] not in ['binary', 'date', 'datetime']
             ])
             tracked_field_names = model._track_get_fields() if 'mail.thread' in model._inherit else []
-            for fname in tracked_field_names:
-                if fname in fields_by_fname:
-                    fields_by_fname[fname]['tracking'] = True
-            for fname, value in default_values_by_fname.items():
-                if fname in fields_by_fname:
-                    fields_by_fname[fname]['default'] = value
-            fields_by_model_names[model_name] = fields_by_fname
+            for fname, field_data in fields_data_by_fname.items():
+                if fname in tracked_field_names:
+                    field_data['tracking'] = True
+                if fname in default_values_by_fname:
+                    field_data['default'] = default_values_by_fname[fname]
+                if fname in model._fields:
+                    inverse_fields = [
+                        field for field in model.pool.field_inverses[model._fields[fname]]
+                        if field.model_name in model_names_to_fetch
+                    ]
+                    if inverse_fields:
+                        field_data['inverse_fname_by_model_name'] = {field.model_name: field.name for field in inverse_fields}
+                    if field_data['type'] == 'many2one_reference':
+                        field_data['model_name_ref_fname'] = model._fields[fname].model_field
+            fields_by_model_names[model_name] = fields_data_by_fname
         return fields_by_model_names
diff --git a/addons/mail/static/tests/helpers/mock_server.js b/addons/mail/static/tests/helpers/mock_server.js
index 254466b48e0..2e617f6f0de 100644
--- a/addons/mail/static/tests/helpers/mock_server.js
+++ b/addons/mail/static/tests/helpers/mock_server.js
@@ -519,13 +519,7 @@ MockServer.include({
             return res;
         }
         if (request_list.includes('activities')) {
-            const activities = this._mockSearchRead('mail.activity', [[
-                '|',
-                    ['id', 'in', thread.activity_ids || []],
-                    '&',
-                        ['res_id', '=', thread.id],
-                        ['res_model', '=', thread_model],
-            ]], {});
+            const activities = this._mockSearchRead('mail.activity', [[['id', 'in', thread.activity_ids || []]]], {});
             res['activities'] = this._mockMailActivityActivityFormat(activities.map(activity => activity.id));
         }
         if (request_list.includes('attachments')) {
@@ -535,13 +529,7 @@ MockServer.include({
             res['attachments'] = this._mockIrAttachment_attachmentFormat(attachments.map(attachment => attachment.id), true);
         }
         if (request_list.includes('followers')) {
-            const followers = this._mockSearchRead('mail.followers', [[
-                '|',
-                    ['id', 'in', thread.message_follower_ids || []],
-                    '&',
-                        ['res_id', '=', thread.id],
-                        ['res_model', '=', thread_model],
-            ]], {});
+            const followers = this._mockSearchRead('mail.followers', [[['id', 'in', thread.message_follower_ids || []]]], {});
             // search read returns many2one relations as an array [id, display_name].
             // But the original route does not. Thus, we need to change it now.
             followers.forEach(follower => follower.partner_id = follower.partner_id[0]);
diff --git a/addons/mail/static/tests/qunit_suite_tests/components/activity_mark_done_popover_tests.js b/addons/mail/static/tests/qunit_suite_tests/components/activity_mark_done_popover_tests.js
index 4d61f7da3f1..3b102c21717 100644
--- a/addons/mail/static/tests/qunit_suite_tests/components/activity_mark_done_popover_tests.js
+++ b/addons/mail/static/tests/qunit_suite_tests/components/activity_mark_done_popover_tests.js
@@ -15,10 +15,7 @@ QUnit.module('activity_mark_done_popover_tests.js', {
 QUnit.test('activity mark done popover simplest layout', async function (assert) {
     assert.expect(6);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
@@ -68,10 +65,7 @@ QUnit.test('activity mark done popover simplest layout', async function (assert)
 QUnit.test('activity with force next mark done popover simplest layout', async function (assert) {
     assert.expect(6);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
@@ -122,10 +116,7 @@ QUnit.test('activity with force next mark done popover simplest layout', async f
 QUnit.test('activity mark done popover mark done without feedback', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
@@ -167,10 +158,7 @@ QUnit.test('activity mark done popover mark done without feedback', async functi
 QUnit.test('activity mark done popover mark done with feedback', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
@@ -221,10 +209,7 @@ QUnit.test('activity mark done popover mark done and schedule next', async funct
         assert.step('activity_action');
         throw new Error("The do-action event should not be triggered when the route doesn't return an action");
     });
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
@@ -279,10 +264,7 @@ QUnit.test('[technical] activity mark done & schedule next with new action', asy
             "The content of the action should be correct"
         );
     });
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'not_upload_file',
         can_write: true,
diff --git a/addons/mail/static/tests/qunit_suite_tests/components/activity_tests.js b/addons/mail/static/tests/qunit_suite_tests/components/activity_tests.js
index bd59a7d2eb4..5fde28978a9 100644
--- a/addons/mail/static/tests/qunit_suite_tests/components/activity_tests.js
+++ b/addons/mail/static/tests/qunit_suite_tests/components/activity_tests.js
@@ -16,10 +16,7 @@ QUnit.module('activity_tests.js', {
 QUnit.test('activity simplest layout', async function (assert) {
     assert.expect(12);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         id: 12,
         res_id: 100,
@@ -95,10 +92,7 @@ QUnit.test('activity simplest layout', async function (assert) {
 QUnit.test('activity with note layout', async function (assert) {
     assert.expect(3);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         id: 12,
         note: 'There is no good or bad note',
@@ -133,10 +127,7 @@ QUnit.test('activity info layout when planned after tomorrow', async function (a
     const today = new Date();
     const fiveDaysFromNow = new Date();
     fiveDaysFromNow.setDate(today.getDate() + 5);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         date_deadline: date_to_str(fiveDaysFromNow),
         id: 12,
@@ -176,10 +167,7 @@ QUnit.test('activity info layout when planned tomorrow', async function (assert)
     const today = new Date();
     const tomorrow = new Date();
     tomorrow.setDate(today.getDate() + 1);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         date_deadline: date_to_str(tomorrow),
         id: 12,
@@ -216,10 +204,7 @@ QUnit.test('activity info layout when planned tomorrow', async function (assert)
 QUnit.test('activity info layout when planned today', async function (assert) {
     assert.expect(4);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         date_deadline: date_to_str(new Date()),
         id: 12,
@@ -259,10 +244,7 @@ QUnit.test('activity info layout when planned yesterday', async function (assert
     const today = new Date();
     const yesterday = new Date();
     yesterday.setDate(today.getDate() - 1);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         date_deadline: date_to_str(yesterday),
         id: 12,
@@ -302,10 +284,7 @@ QUnit.test('activity info layout when planned before yesterday', async function
     const today = new Date();
     const fiveDaysBeforeNow = new Date();
     fiveDaysBeforeNow.setDate(today.getDate() - 5);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         date_deadline: date_to_str(fiveDaysBeforeNow),
         id: 12,
@@ -342,10 +321,7 @@ QUnit.test('activity info layout when planned before yesterday', async function
 QUnit.test('activity with a summary layout', async function (assert) {
     assert.expect(4);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         id: 12,
         res_id: 100,
@@ -382,10 +358,7 @@ QUnit.test('activity with a summary layout', async function (assert) {
 QUnit.test('activity without summary layout', async function (assert) {
     assert.expect(5);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_type_id: 1,
         id: 12,
@@ -430,10 +403,7 @@ QUnit.test('activity details toggle', async function (assert) {
     const today = new Date();
     const tomorrow = new Date();
     tomorrow.setDate(today.getDate() + 1);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         create_date: date_to_str(today),
         create_uid: 1,
@@ -492,10 +462,7 @@ QUnit.test('activity details layout', async function (assert) {
         id: 10,
         name: 'Pauvre pomme',
     });
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_type_id: 1,
         create_date: date_to_str(today),
@@ -576,10 +543,7 @@ QUnit.test('activity details layout', async function (assert) {
 QUnit.test('activity with mail template layout', async function (assert) {
     assert.expect(8);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.template'].records.push({
         id: 1,
         name: "Dummy mail template",
@@ -675,10 +639,7 @@ QUnit.test('activity with mail template: preview mail', async function (assert)
         );
     });
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 42,
-    });
+    this.data['res.partner'].records.push({ id: 42 });
     this.data['mail.template'].records.push({
         id: 1,
         name: "Dummy mail template",
@@ -716,10 +677,7 @@ QUnit.test('activity with mail template: preview mail', async function (assert)
 QUnit.test('activity with mail template: send mail', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 42,
-    });
+    this.data['res.partner'].records.push({ id: 42 });
     this.data['mail.template'].records.push({
         id: 1,
         name: "Dummy mail template",
@@ -770,10 +728,7 @@ QUnit.test('activity with mail template: send mail', async function (assert) {
 QUnit.test('activity upload document is available', async function (assert) {
     assert.expect(3);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100, });
     this.data['mail.activity'].records.push({
         activity_category: 'upload_file',
         activity_type_id: 28,
@@ -806,10 +761,7 @@ QUnit.test('activity upload document is available', async function (assert) {
 QUnit.test('activity click on mark as done', async function (assert) {
     assert.expect(4);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'default',
         activity_type_id: 1,
@@ -856,10 +808,7 @@ QUnit.test('activity click on mark as done', async function (assert) {
 QUnit.test('activity mark as done popover should focus feedback input on open [REQUIRE FOCUS]', async function (assert) {
     assert.expect(3);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'default',
         activity_type_id: 1,
@@ -927,10 +876,7 @@ QUnit.test('activity click on edit', async function (assert) {
         );
     });
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 42,
-    });
+    this.data['res.partner'].records.push({ id: 42 });
     this.data['mail.template'].records.push({
         id: 1,
         name: "Dummy mail template",
@@ -969,10 +915,7 @@ QUnit.test('activity click on edit', async function (assert) {
 QUnit.test('activity edition', async function (assert) {
     assert.expect(14);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 42,
-    });
+    this.data['res.partner'].records.push({ id: 42 });
     this.data['mail.activity'].records.push({
         can_write: true,
         icon: 'fa-times',
@@ -1064,10 +1007,7 @@ QUnit.test('activity edition', async function (assert) {
 QUnit.test('activity click on cancel', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_type_id: 1,
         can_write: true,
@@ -1121,10 +1061,7 @@ QUnit.test('activity mark done popover close on ESCAPE', async function (assert)
     // This test is not in activity_mark_done_popover_tests.js as it requires the activity mark done
     // component to have a parent in order to allow testing interactions the popover.
     assert.expect(2);
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'default',
         activity_type_id: 1,
@@ -1164,10 +1101,7 @@ QUnit.test('activity mark done popover click on discard', async function (assert
     // component to have a parent in order to allow testing interactions the popover.
     assert.expect(3);
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'default',
         activity_type_id: 1,
@@ -1228,10 +1162,7 @@ QUnit.test('data-oe-id & data-oe-model link redirection on click', async functio
         assert.step('do-action:openFormView_some.model_250');
     });
 
-    this.data['res.partner'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'default',
         activity_type_id: 1,
@@ -1268,10 +1199,7 @@ QUnit.test('button related to file uploading is replaced when updating activity
     assert.expect(2);
 
     const activityId = 513;
-    this.data['res.partner'].records.push({
-        activity_ids: [activityId],
-        id: 100,
-    });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         activity_category: 'upload_file',
         activity_type_id: 28,
diff --git a/addons/mail/static/tests/qunit_suite_tests/components/follow_button_tests.js b/addons/mail/static/tests/qunit_suite_tests/components/follow_button_tests.js
index e95e99e8cfe..3b8d217bd08 100644
--- a/addons/mail/static/tests/qunit_suite_tests/components/follow_button_tests.js
+++ b/addons/mail/static/tests/qunit_suite_tests/components/follow_button_tests.js
@@ -76,7 +76,7 @@ QUnit.test('base rendering editable', async function (assert) {
 QUnit.test('hover following button', async function (assert) {
     assert.expect(8);
 
-    this.data['res.partner'].records.push({ id: 100, message_follower_ids: [1] });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.followers'].records.push({
         id: 1,
         is_active: true,
@@ -143,7 +143,7 @@ QUnit.test('hover following button', async function (assert) {
 QUnit.test('click on "follow" button', async function (assert) {
     assert.expect(6);
 
-    this.data['res.partner'].records.push({ id: 100, message_follower_ids: [1] });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.followers'].records.push({
         id: 1,
         is_active: true,
@@ -197,7 +197,7 @@ QUnit.test('click on "follow" button', async function (assert) {
 QUnit.test('click on "unfollow" button', async function (assert) {
     assert.expect(7);
 
-    this.data['res.partner'].records.push({ id: 100, message_follower_ids: [1] });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.followers'].records.push({
         id: 1,
         is_active: true,
diff --git a/addons/mail/static/tests/qunit_suite_tests/components/follower_tests.js b/addons/mail/static/tests/qunit_suite_tests/components/follower_tests.js
index a7bccbaa53d..957297fd970 100644
--- a/addons/mail/static/tests/qunit_suite_tests/components/follower_tests.js
+++ b/addons/mail/static/tests/qunit_suite_tests/components/follower_tests.js
@@ -187,7 +187,7 @@ QUnit.test('click on partner follower details', async function (assert) {
 QUnit.test('click on edit follower', async function (assert) {
     assert.expect(5);
 
-    this.data['res.partner'].records.push({ id: 100, message_follower_ids: [2] });
+    this.data['res.partner'].records.push({ id: 100 });
     this.data['mail.followers'].records.push({
         id: 2,
         is_active: true,
diff --git a/addons/web/__manifest__.py b/addons/web/__manifest__.py
index 309545a97a6..81ede0229df 100644
--- a/addons/web/__manifest__.py
+++ b/addons/web/__manifest__.py
@@ -490,6 +490,7 @@ This module provides the core of the Odoo Web Client.
             'web/static/tests/webclient/**/helpers.js',
             'web/static/tests/qunit.js',
             'web/static/tests/main.js',
+            'web/static/tests/mock_relational_fields_tests.js',
             'web/static/tests/mock_server_tests.js',
             'web/static/tests/setup.js',
 
diff --git a/addons/web/static/tests/legacy/helpers/mock_server.js b/addons/web/static/tests/legacy/helpers/mock_server.js
index 64ef35c9fcf..d10e54c8e8c 100644
--- a/addons/web/static/tests/legacy/helpers/mock_server.js
+++ b/addons/web/static/tests/legacy/helpers/mock_server.js
@@ -52,6 +52,10 @@ var MockServer = Class.extend({
             }
         }
 
+        // fill relational fields' inverse.
+        for (const modelName in this.data) {
+            this.data[modelName].records.forEach(record => this._updateComodelRelationalFields(modelName, record.id, record));
+        }
         this.debug = options.debug;
 
         this.currentDate = options.currentDate || moment().format("YYYY-MM-DD");
@@ -578,6 +582,7 @@ var MockServer = Class.extend({
         model.records.push(record);
         this._applyDefaults(model, values);
         this._writeRecord(modelName, values, id);
+        this._updateComodelRelationalFields(modelName, id, values);
         return id;
     },
     /**
@@ -1846,14 +1851,18 @@ var MockServer = Class.extend({
             return _.contains(ids, record.id);
         });
 
-        // update value of one2many fields pointing to the deleted records
+        // update value of relationnal fields pointing to the deleted records
         _.each(this.data, function (d) {
             var relatedFields = _.pick(d.fields, function (field) {
-                return field.type === 'one2many' && field.relation === model;
+                return field.relation === model;
             });
             _.each(Object.keys(relatedFields), function (relatedField) {
                 _.each(d.records, function (record) {
-                    record[relatedField] = _.difference(record[relatedField], ids);
+                    if (Array.isArray(record[relatedField])) {
+                        record[relatedField] = _.difference(record[relatedField], ids);
+                    } else if (ids.includes(record[relatedField])) {
+                        record[relatedField] = false;
+                    }
                 });
             });
         });
@@ -1914,7 +1923,12 @@ var MockServer = Class.extend({
      * @returns {boolean} currently, always return 'true'
      */
     _mockWrite: function (model, args) {
-        _.each(args[0], this._writeRecord.bind(this, model, args[1]));
+        _.each(args[0], id => {
+            const originalRecord = this._mockSearchRead(model, [[['id', '=', id]]], {})[0];
+            this._writeRecord(model, args[1], id);
+            const updatedRecord = this.data[model].records.find(record => record.id === id);
+            this._updateComodelRelationalFields(model, id, updatedRecord, originalRecord);
+        });
         return true;
     },
     /**
@@ -2076,6 +2090,66 @@ var MockServer = Class.extend({
             _.each(tree.childNodes, function (c) { self._traverse(c, f); });
         }
     },
+    /**
+     * Fill all inverse fields of the relational fields present in the record
+     * to be created/updated.
+     *
+     * @param {string} modelName
+     * @param {Object} record record that have been created/updated.
+     * @param {Object|undefined} originalRecord record before update.
+     */
+     _updateComodelRelationalFields(modelName, recordId, record, originalRecord) {
+        for (const fname in record) {
+            const field = this.data[modelName].fields[fname];
+            const comodelName = field.relation || record[field['model_name_ref_fname']];
+            const inverseFieldName = field['inverse_fname_by_model_name'] && field['inverse_fname_by_model_name'][comodelName];
+            if (!inverseFieldName) {
+                // field has no inverse, skip it.
+                continue;
+            }
+            const relatedRecordIds = Array.isArray(record[fname]) ? record[fname] : [record[fname]];
+            // we only want to set a value for comodel inverse field if the model field has a value.
+            if (record[fname]) {
+                for (const relatedRecordId of relatedRecordIds) {
+                    let inverseFieldNewValue = recordId;
+                    const relatedRecord = this.data[comodelName].records.find(record => record.id === relatedRecordId);
+                    const relatedFieldValue = relatedRecord && relatedRecord[inverseFieldName];
+                    if (
+                        relatedFieldValue === undefined ||
+                        relatedFieldValue === recordId ||
+                        field.type !== 'one2many' && relatedFieldValue.includes(recordId)
+                    ) {
+                        // related record does not exist or the related value is already up to date.
+                        continue;
+                    }
+                    if (Array.isArray(relatedFieldValue)) {
+                        inverseFieldNewValue = [...relatedFieldValue, recordId];
+                    }
+                    this._writeRecord(comodelName, { [inverseFieldName]: inverseFieldNewValue }, relatedRecordId);
+                }
+            } else if (field.type === 'many2one_reference') {
+                // we need to clean the many2one_field as well.
+                const comodel_inverse_field = this.data[comodelName].fields[inverseFieldName];
+                const model_many2one_field = comodel_inverse_field['inverse_fname_by_model_name'][modelName];
+                this._writeRecord(modelName, { [model_many2one_field]: false }, recordId);
+            }
+            // it's an update, get the records that were originally referenced but are not
+            // anymore and update their relational fields.
+            if (originalRecord) {
+                const originalRecordIds = Array.isArray(originalRecord[fname]) ? originalRecord[fname] : [originalRecord[fname]];
+                // search read returns [id, name], let's ensure the removedRecordIds are integers.
+                const removedRecordIds = originalRecordIds.filter(recordId => Number.isInteger(recordId) && !relatedRecordIds.includes(recordId));
+                for (const removedRecordId of removedRecordIds) {
+                    const removedRecord = this.data[comodelName].records.find(record => record.id === removedRecordId);
+                    let inverseFieldNewValue = false;
+                    if (Array.isArray(removedRecord[inverseFieldName])) {
+                        inverseFieldNewValue = removedRecord[inverseFieldName].filter(id => id !== recordId);
+                    }
+                    this._writeRecord(comodelName, { [inverseFieldName]: inverseFieldNewValue }, removedRecordId);
+                }
+            }
+        }
+    },
     /**
      * Write a record. The main difficulty is that we have to apply x2many
      * commands
diff --git a/addons/web/static/tests/mock_relational_fields_tests.js b/addons/web/static/tests/mock_relational_fields_tests.js
new file mode 100644
index 00000000000..4c2ce9060f8
--- /dev/null
+++ b/addons/web/static/tests/mock_relational_fields_tests.js
@@ -0,0 +1,135 @@
+/** @odoo-module **/
+
+import MockServer from 'web.MockServer';
+
+QUnit.module('web', {}, function () {
+QUnit.module('mock_relational_fields_tests.js', {
+    beforeEach() {
+        this.data = {
+            foo: {
+                fields: {
+                    one2many_field: { type: 'one2many', relation: 'bar', inverse_fname_by_model_name: { bar: 'many2one_field' } },
+                    many2one_field: { type: 'many2one', relation: 'bar', inverse_fname_by_model_name: { bar: 'one2many_field' } },
+                    many2many_field: { type: 'many2many', relation: 'bar', inverse_fname_by_model_name: { bar: 'many2many_field' } },
+                    many2one_reference: { type: 'many2one_reference', model_name_ref_fname: 'res_model', inverse_fname_by_model_name: { bar: 'one2many_field' } },
+                    res_model: { type: 'char' },
+
+                },
+                records: [],
+            },
+            bar: {
+                fields: {
+                    many2one_field: { type: 'many2one', relation: 'foo' },
+                    one2many_field: { type: 'one2many', relation: 'foo', inverse_fname_by_model_name: { foo: 'many2one_field' } },
+                    many2many_field: { type: 'many2many', relation: 'foo', inverse_fname_by_model_name: { foo: 'many2many_field' } },
+                },
+                records: [],
+            },
+        };
+    }
+});
+
+QUnit.test('many2one_ref should auto fill inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        res_model: 'bar',
+        many2one_reference: 1,
+    });
+    const mockServer = new MockServer(this.data, {});
+    assert.deepEqual([2], mockServer.data['bar'].records[0].one2many_field);
+
+    mockServer._mockUnlink('foo', [2]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].one2many_field);
+});
+
+QUnit.test('many2one should auto fill inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        many2one_field: 1,
+    });
+    const mockServer = new MockServer(this.data, {});
+    assert.deepEqual([2], mockServer.data['bar'].records[0].one2many_field);
+
+    mockServer._mockUnlink('foo', [2]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].one2many_field);
+});
+
+QUnit.test('one2many should auto fill inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['bar'].records.push({ id: 2 });
+    this.data['foo'].records.push({
+        id: 3,
+        one2many_field: [1, 2],
+    });
+    const mockServer = new MockServer(this.data, {});
+    assert.strictEqual(3, mockServer.data['bar'].records[0].many2one_field);
+    assert.strictEqual(3, mockServer.data['bar'].records[1].many2one_field);
+
+    mockServer._mockUnlink('foo', [3]);
+    assert.strictEqual(false, mockServer.data['bar'].records[0].many2one_field);
+    assert.strictEqual(false, mockServer.data['bar'].records[1].many2one_field);
+});
+
+QUnit.test('many2many should auto fill inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        many2many_field: [1],
+    });
+    const mockServer = new MockServer(this.data, {});
+    assert.deepEqual([2], mockServer.data['bar'].records[0].many2many_field);
+
+    mockServer._mockUnlink('foo', [2]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].many2many_field);
+});
+
+QUnit.test('one2many update should update inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['bar'].records.push({ id: 2 });
+    this.data['foo'].records.push({
+        id: 3,
+        one2many_field: [1, 2],
+    });
+    const mockServer = new MockServer(this.data, {});
+    mockServer._mockWrite('foo', [[3], { one2many_field: [1] }]);
+    assert.strictEqual(3, mockServer.data['bar'].records[0].many2one_field);
+    assert.strictEqual(false, mockServer.data['bar'].records[1].many2one_field);
+});
+
+QUnit.test('many2many update should update inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        many2many_field: [1],
+    });
+    const mockServer = new MockServer(this.data, {});
+    mockServer._mockWrite('foo', [[2], { many2many_field: [] }]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].many2many_field);
+});
+
+QUnit.test('many2one update should update inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        many2one_field: 1,
+    });
+    const mockServer = new MockServer(this.data, {});
+    mockServer._mockWrite('foo', [[2], { many2one_field: false }]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].one2many_field);
+});
+
+QUnit.test('many2one_ref update should update inverse field', async function (assert) {
+    this.data['bar'].records.push({ id: 1 });
+    this.data['foo'].records.push({
+        id: 2,
+        res_model: 'bar',
+        many2one_reference: 1,
+    });
+    const mockServer = new MockServer(this.data, {});
+    mockServer._mockWrite('foo', [[2], { many2one_reference: false }]);
+    assert.deepEqual([], mockServer.data['bar'].records[0].one2many_field);
+});
+
+});
diff --git a/addons/website_slides/static/tests/qunit_suite_tests/components/activity_tests.js b/addons/website_slides/static/tests/qunit_suite_tests/components/activity_tests.js
index e8219ca237f..794a6370c26 100644
--- a/addons/website_slides/static/tests/qunit_suite_tests/components/activity_tests.js
+++ b/addons/website_slides/static/tests/qunit_suite_tests/components/activity_tests.js
@@ -14,10 +14,7 @@ QUnit.test('grant course access', async function (assert) {
     assert.expect(8);
 
     this.data['res.partner'].records.push({ id: 5 });
-    this.data['slide.channel'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['slide.channel'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         can_write: true,
         id: 12,
@@ -54,10 +51,7 @@ QUnit.test('refuse course access', async function (assert) {
     assert.expect(8);
 
     this.data['res.partner'].records.push({ id: 5 });
-    this.data['slide.channel'].records.push({
-        activity_ids: [12],
-        id: 100,
-    });
+    this.data['slide.channel'].records.push({ id: 100 });
     this.data['mail.activity'].records.push({
         can_write: true,
         id: 12,