Fixes setAssociation with scope (#7159)

* Fixes setAssociation with scope

* Fixes style

lib/associations/belongs-to-many.js

@@ -583,6 +583,8 @@ class BelongsToMany extends Association {
     }
 
     where[identifier] = sourceInstance.get(sourceKey);
+    _.assign(where, association.through.scope);
+
     return association.through.model.findAll(_.defaults({where, raw: true}, options)).then(currentRows => {
       const obsoleteAssociations = [];
       const promises = [];

test/integration/associations/belongs-to-many.test.js

@@ -616,6 +616,56 @@ describe(Support.getTestDialectTeaser('BelongsToMany'), function() {
         expect(tasks[0].title).to.equal('wat');
       });
     });
+
+    it('using scope to set associations', function() {
+      var ItemTag = this.sequelize.define('ItemTag', {
+                      id : { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
+                      tag_id: { type: DataTypes.INTEGER, unique: false },
+                      taggable: { type: DataTypes.STRING },
+                      taggable_id: { type: DataTypes.INTEGER, unique: false }
+      }),
+      Tag = this.sequelize.define('Tag', {
+        id : { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
+        name: DataTypes.STRING
+      }),
+      Comment = this.sequelize.define('Comment', {
+        id : { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
+        name: DataTypes.STRING
+      }),
+      Post = this.sequelize.define('Post', {
+        id : { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
+        name: DataTypes.STRING
+      });
+
+      Post.belongsToMany(Tag, {
+        through: { model: ItemTag, unique: false, scope: { taggable: 'post' } },
+        foreignKey: 'taggable_id'
+      });
+
+      Comment.belongsToMany(Tag, {
+        through: { model: ItemTag, unique: false, scope: { taggable: 'comment' } },
+       foreignKey: 'taggable_id'
+      });
+
+      return this.sequelize.sync({ force: true }).then(function() {
+        return Promise.all([
+          Post.create({ name: 'post1' }),
+          Comment.create({ name: 'comment1' }),
+          Tag.create({ name: 'tag1' })
+        ]);
+      }).bind({}).spread(function(post, comment, tag) {
+        this.post = post;
+        this.comment = comment;
+        this.tag = tag;
+        return this.post.setTags([this.tag]);
+      }).then(function() {
+        return this.comment.setTags([this.tag]);
+      }).then(function() {
+        return this.comment.getTags();
+      }).then(function(_tags) {
+        expect(_tags).to.have.length(1);
+      });
+    });
   });
 
   describe('createAssociations', function() {