title: Prevent model creation when building associations with Waterline
body: <p>I have two model, ModelA and ModelB, with a one-to-many association : ModelA owns a lot of ModelB.</p>
  
  <pre><code>// ModelA
  module.exports = {
      attributes: {
          model_b: {
              collection: ModelB
          }
      }
  };
  </code></pre>
  
  <hr>
  
  <pre><code>// ModelB
  module.exports = {
      attributes: {
          model_a: {
              model: ModelA,
              required: true
          }
      }
  };
  </code></pre>
  
  <p>When I create a new ModelB, I have to assign it to an <em>existing</em> ModelA. However, when I give a ModelA object as a parameter to the ModelB's create function, it doesn't bother if the ModelA exists or not : it tries to create a new one.</p>
  
  <p>I tried to fix this with a beforeValidate function which replace the given ModelA object by its id if it exists and throws an error if it doesn't. This function isn't even called...</p>
  
  <p>Have someone already faced this kind of issue ? How should I handle this ?</p>
  
  <p>Thanks for reading.</p>