get foreignKey() {
    return this.getForeignKey();
  }
Returns the name used for the association's foreign key.

    ```js
    let server = createServer({
      models: {
        user: Model,
        post: Model.extend({
          fineAuthor: belongsTo("user"),
          comments: hasMany()
        }),
        comment: Model
      }
    });

    let associations = server.associationsFor('post')

    associations.fineAuthor.foreignKey // fineAuthorId
    associations.comments.foreignKey // commentIds
    ```
  
    @accessor
    @type {String}
    @public