fix(associations): throw informational message when try to associate undefined (#8106)

lib/associations/mixin.js

@@ -9,7 +9,7 @@ const BelongsTo = require('./belongs-to');
 
 const Mixin = {
   hasMany(target, options) { // testhint options:none
-    if (!target.prototype || !(target.prototype instanceof this.sequelize.Model)) {
+    if (!target || !target.prototype || !(target.prototype instanceof this.sequelize.Model)) {
       throw new Error(this.name + '.hasMany called with something that\'s not a subclass of Sequelize.Model');
     }
 
@@ -33,7 +33,7 @@ const Mixin = {
   },
 
   belongsToMany(targetModel, options) { // testhint options:none
-    if (!targetModel.prototype || !(targetModel.prototype instanceof this.sequelize.Model)) {
+    if (!targetModel || !targetModel.prototype || !(targetModel.prototype instanceof this.sequelize.Model)) {
       throw new Error(this.name + '.belongsToMany called with something that\'s not a subclass of Sequelize.Model');
     }
 
@@ -77,7 +77,7 @@ const Mixin = {
 // The logic for hasOne and belongsTo is exactly the same
 function singleLinked(Type) {
   return function(target, options) { // testhint options:none
-    if (!target.prototype || !(target.prototype instanceof this.sequelize.Model)) {
+    if (!target || !target.prototype || !(target.prototype instanceof this.sequelize.Model)) {
       throw new Error(this.name + '.' + Utils.lowercaseFirst(Type.toString()) + ' called with something that\'s not a subclass of Sequelize.Model');
     }