morphMany(Target, name, columnNames, morphValue) {
      return this._morphOneOrMany(Target, name, columnNames, morphValue, 'morphMany');
    }
{@link Model#morphMany morphMany} is essentially the same as a {@link
Model#morphOne morphOne}, but creating a {@link Collection collection}
rather than a {@link Model model} (similar to a {@link Model#hasOne
hasOne} vs. {@link Model#hasMany hasMany} relation).

{@link Model#morphMany morphMany} is used to signify a {@link oneToMany
one-to-many} or {@link manyToMany many-to-many} {@link polymorphicRelation
polymorphic relation} with another `Target` model, where the `name` of the
model is used to determine which database table keys are used. The naming
convention requires the `name` prefix an `_id` and `_type` field in the
database. So for the case below the table names would be `imageable_type`
and `imageable_id`. The `morphValue` may be optionally set to
store/retrieve a different value in the `_type` column than the `Target`'s
{@link Model#tableName tableName}.

    let Post = bookshelf.model('Post', {
      tableName: 'posts',
      photos: function() {
        return this.morphMany('Photo', 'imageable');
      }
    });

And with custom columnNames:

    let Post = bookshelf.model('Post'{
      tableName: 'posts',
      photos: function() {
        return this.morphMany('Photo', 'imageable', ['ImageableType', 'ImageableId']);
      }
    });

@method Model#morphMany
@param {Model|string} Target
  Constructor of {@link Model} targeted by join. Can be a string specifying a previously registered model with
  {@link Bookshelf#model}.
@param {string} [name] Prefix for `_id` and `_type` columns.
@param {(string[])} [columnNames]
  Array containing two column names, the first is the `_type` while the second is the `_id`.
@param {string} [morphValue=Target#{@link Model#tableName tablename}]
  The string value associated with this relationship. Stored in the `_type` column of the polymorphic table.
  Defaults to `Target`#{@link Model#tableName tablename}.
@returns {Collection} A collection of related models.