fix(btm): Expose belongs to many getter similar to other associations (#6552)

lib/associations/belongs-to-many.js

@@ -421,12 +421,10 @@ class BelongsToMany extends Association {
     return this;
   }
 
-  injectGetter(obj) {
-    const association = this;
-
-    obj[this.accessors.get] = function(options) {
+  get(instance, options) {
     options = Utils.cloneDeep(options) || {};
 
+    const association = this;
     const through = association.through;
     let scopeWhere;
     let throughWhere;
@@ -444,7 +442,7 @@ class BelongsToMany extends Association {
 
     if (Object(through.model) === through.model) {
       throughWhere = {};
-        throughWhere[association.foreignKey] = this.get(association.source.primaryKeyAttribute);
+      throughWhere[association.foreignKey] = instance.get(association.source.primaryKeyAttribute);
 
       if (through.scope) {
         _.assign(throughWhere, through.scope);
@@ -480,6 +478,13 @@ class BelongsToMany extends Association {
     }
 
     return model.findAll(options);
+  }
+
+  injectGetter(obj) {
+    const association = this;
+
+    obj[this.accessors.get] = function(options) {
+      return association.get(this, options);
     };
 
     obj[this.accessors.count] = function(options) {