code: function(input)
    {
      var key = this.buildKeyFromInput( input );
      var index = this.map.indices[ key ];
  
      return index === undefined ? -1 : index;
    }
docs: Returns the index of the given model in this collection or returns -1
  if the model doesn't exist in this collection.
  
  @method
  @memberof Rekord.ModelCollection#
  @param {modelInput} input -
  The model to search for.
  @return {Number} -
  The index of the model in this collection or -1 if it was not found.