[FIX] web: fix one2many default get

The one2many default get code used to meddle with basic_model's res_ids
manually, which ended up desynchronizing the data and res_ids fields
of the list representation of the one2many values.

This commit solves the issue by using the proper mechanisms of the
basic_model, that is creating a proper datapoint for those ids.

Steps to reproduce the original issue:
- Install sale_mamangement, mrp and purchase
- Create a product
- In the Inventory tab, in Routes, uncheck Buy
- Check and then uncheck back any other route than Buy
- Get a traceback because we are looking for a record with a given
  res_id in the list data but since they are desynchronized, data
  is empty at that point even though res_ids is not.

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

@@ -3158,8 +3158,18 @@ var BasicModel = AbstractModel.extend({
                             }
                             if (value[0] === 6) {
                                 // REPLACE_WITH
-                                x2manyList.res_ids = value[2];
-                                x2manyList.count = x2manyList.res_ids.length;
+                                _.each(value[2], function (res_id) {
+                                    r = self._makeDataPoint({
+                                        modelName: x2manyList.model,
+                                        context: x2manyList.context,
+                                        fieldsInfo: fieldsInfo,
+                                        fields: fields,
+                                        parentID: x2manyList.id,
+                                        res_id: res_id,
+                                        viewType: viewType,
+                                    });
+                                    x2manyList._changes.push({operation: 'ADD', id: r.id});
+                                });
                                 var def = self._readUngroupedList(x2manyList).then(function () {
                                     return $.when(
                                         self._fetchX2ManysBatched(x2manyList),

addons/web/static/tests/fields/relational_fields_tests.js

@@ -6249,6 +6249,53 @@ QUnit.module('relational_fields', {
         form.destroy();
     });
 
+    QUnit.test('many2many checkboxes with default values', function (assert) {
+        assert.expect(7);
+
+        this.data.partner.fields.turtles.default = [3];
+        this.data.partner.fields.turtles.type = "many2many";
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch:'<form string="Partners">' +
+                    '<field name="turtles" widget="many2many_checkboxes">' +
+                    '</field>' +
+                '</form>',
+            mockRPC: function (route, args) {
+                if (args.method === 'create') {
+                    assert.deepEqual(args.args[0].turtles, [[6, false, [1]]],
+                        "correct values should have been sent to create");
+                }
+                return this._super.apply(this, arguments);
+            }
+        });
+
+        assert.notOk(form.$('.o_checkbox input').eq(0).prop('checked'),
+            "first checkbox should not be checked");
+        assert.notOk(form.$('.o_checkbox input').eq(1).prop('checked'),
+            "second checkbox should not be checked");
+        assert.ok(form.$('.o_checkbox input').eq(2).prop('checked'),
+            "third checkbox should be checked");
+
+        form.$('.o_checkbox input:checked').click();  // Uncheck default record
+        form.$('.o_checkbox input').first().click();  // Check first record
+        form.$('.o_checkbox input').first().click();  // Uncheck first record
+        form.$('.o_checkbox input').first().click();  // Recheck first record
+
+        assert.ok(form.$('.o_checkbox input').eq(0).prop('checked'),
+            "first checkbox should be checked");
+        assert.notOk(form.$('.o_checkbox input').eq(1).prop('checked'),
+            "second checkbox should not be checked");
+        assert.notOk(form.$('.o_checkbox input').eq(2).prop('checked'),
+            "third checkbox should not be checked");
+
+        form.$buttons.find('.o_form_button_save').click();
+
+        form.destroy();
+    });
+
     QUnit.test('many2many list with x2many: add a record', function (assert) {
         assert.expect(18);