[FIX] web: basicmodel: properly link parent in m2m

In some cases, the parentID was not properly set, which is a problem
when we need to evaluate the parent in a context.

Note: I also added a better error explanation in the mock server, to
make it better for the developper to see what is wrong.

addons/web/static/src/js/views/basic/basic_model.js

@@ -1168,6 +1168,7 @@ var BasicModel = AbstractModel.extend({
                         fieldsInfo: view ? view.fieldsInfo : fieldInfo.fieldsInfo,
                         res_id: d.id,
                         viewType: view ? view.type : fieldInfo.viewType,
+                        parentID: list.id,
                     });
                     list_records[d.id] = rec;
                     list._changes.push(rec.id);

addons/web/static/tests/helpers/mock_server.js

@@ -196,6 +196,9 @@ var MockServer = Class.extend({
                 // 'transfer_field_to_modifiers' simulation
                 var field = fields[node.attrs.name];
 
+                if (!field) {
+                    throw new Error("Field " + node.attrs.name + " does not exist");
+                }
                 var defaultValues = {};
                 var stateExceptions = {};
                 _.each(modifiersNames, function (attr) {

addons/web/static/tests/views/basic_model_tests.js

@@ -407,6 +407,40 @@ QUnit.module('Views', {
         model.destroy();
     });
 
+    QUnit.test('many2many with ADD_M2M command and context with parent key', function (assert) {
+        assert.expect(1);
+
+        this.data.partner_type.fields.some_char = {type: "char"};
+        this.params.fieldsInfo = {
+            default: {
+                category: {
+                    fieldsInfo: {default: {some_char: { context: "{'a': parent.foo}"}}},
+                    relatedFields: {some_char: {type: "char"}},
+                    viewType: 'default',
+                },
+                foo: {},
+            },
+        };
+
+        var model = createModel({
+            Model: BasicModel,
+            data: this.data,
+        });
+
+        model.load(this.params).then(function (resultID) {
+            var changes = {
+                category: {operation: 'ADD_M2M', ids: [{id: 12}]}
+            };
+            model.notifyChanges(resultID, changes).then(function () {
+                var record = model.get(resultID);
+                var categoryRecord = record.data.category.data[0];
+                assert.deepEqual(categoryRecord.getContext({fieldName: 'some_char'}), {a:'gnap'},
+                    "should properly evaluate context");
+            });
+        });
+        model.destroy();
+    });
+
     QUnit.test('can fetch a list', function (assert) {
         assert.expect(4);