title: Dual relationship to model with one polymorphic
body: <p>I have a scenario where I have two models "group" and "motion".
  A group may have many motions and every motion is related to a group.
  A motion may also be related to one of various models of which group may be one.</p>
  
  <p>So I have a has_many relationship between Group &amp; Motion and a polymorphic relationship between Motion &amp; Group.</p>
  
  <pre><code>class Group &lt; ApplicationRecord
    has_many :motions
    has_many :motions, as: :motionable
  end
  
  class Motion &lt; ApplicationRecord
    belongs_to :group  
    belongs_to :motionable, polymorphic: true
  end
  </code></pre>
  
  <p>Whilst this appears to work a new record in motions creates the polymorphic relationship whereas I require the has_many relationship:</p>
  
  <pre><code>2.4.1 :010 &gt; Group.first.motions.new
    Group Load (1.5ms)  SELECT  "groups".* FROM "groups" ORDER BY "groups"."id" ASC LIMIT $1  [["LIMIT", 1]]
   =&gt; #&lt;Motion id: nil, group_id: nil, motionable_type: "Group", motionable_id: 1&gt;
  </code></pre>