Ignore model-level validation when creating through table using string (#5556)

* Ignore model-level validation when creating join table for n:m associations.

When declaring a join table with the through option declared as a
string, Sequelize creates the through model at runtime, and copies the
model validation rules from the source table into the through model. More
often than not, the model validation rules from the source table would be
inappropriate/undesirable for the join table (since their data fields are
different anyway).

It's also worth noting that if a through model is given, this behavior
is not exhibited.

* Added unit test for issue #5556.

lib/associations/belongs-to-many.js

@@ -161,7 +161,8 @@ var BelongsToMany = function(source, target, options) {
       this.through.model = this.sequelize.define(this.through.model, {}, _.extend(this.options, {
         tableName: this.through.model,
         indexes: {}, //we dont want indexes here (as referenced in #2416)
-        paranoid: false  // A paranoid join table does not make sense
+        paranoid: false,  // A paranoid join table does not make sense
+        validate: {} // Don't propagate model-level validations
       }));
     } else {
       this.through.model = this.sequelize.model(this.through.model);

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

@@ -40,6 +40,24 @@ describe(Support.getTestDialectTeaser('belongsToMany'), function() {
     expect(AB.options.scopes).to.have.length(0);
   });
 
+  it('should not inherit validations from parent to join table', function () {
+    var A = current.define('a')
+      , B = current.define('b', {}, {
+        validate: {
+          validateModel: function () {
+            return true;
+          }
+        }
+      })
+      , AB;
+
+    B.belongsToMany(A, { through: 'AB' });
+
+    AB = current.model('AB');
+
+    expect(AB.options.validate).to.deep.equal({});
+  });
+
   describe('timestamps', function () {
     it('follows the global timestamps true option', function () {
       var User = current.define('User', {})