I cannot replace an Array with my Mongoid based model

[Matthew Hillsborough]

I have a Mongoid based model in my Ruby on Rails application. 

Here's a slimmed down version of the model:

    module MLB

    class Roster

    include Mongoid::Document

    include Mongoid::Timestamps

    

    field :team_id

    field :season_year

    field :position_categories, type: Array

    

    index({team_id: 1})

    

    validates :team_id, presence: true

    validates :season_year, presence: true

    validates :position_categories, presence: true

    

    def self.parse!(entry)

                            // .. cut 

    

    obj = find_or_create_by(team_id: json_document['team_id'], season_year: json_document['season_year'])

    obj.team_id = json_document['team_id']

    obj.season_year = json_document['title'].match(/\d+/)[0].to_i

                        # PROBLEM HERE - new array will not overwrite existing array

    obj.position_categories = json_document['position_categories']

    

    obj.save

    end

        end

The problem I'm having is commented above. I can't just simply overwrite the existing array, position_categories, with the new one as depicted above. Do I need to empty it out first? Is there an easy way to just overwrite the entire array here? For what it's worth, I'm using Mongoid 4.

I've also tried doing the following with NO luck:

obj.position_categories = obj.position_categories || []

obj.position_categories.replace(json_document['position_categories'])

The original array just stays in place. I have to delete the document entirely and re-create it for the new position_categories to take place. Any ideas on what's going on here?