get isPolymorphic() {
    return this.opts.polymorphic;
  }
Returns a Boolean that's true if the association is polymorphic:

     For example, given

     ```js
     createServer({
       models: {
         comment: Model.extend({
           commentable: belongsTo({ polymorphic: true })
         })
       }
     })
     ```

     then

     ```js
     server.schema.associationsFor('comment').commentable.isPolymorphic // true
     ```

     Check out [the guides on polymorphic associations](https://miragejs.com/docs/main-concepts/relationships/#polymorphic) to learn more.

     @accessor isPolymorphic
     @type {Boolean}
     @public