discourse/spec/serializers/category_serializer_spec.rb

require 'rails_helper'
require_dependency 'category'

describe CategorySerializer do
  let(:category) { Fabricate(:category) }

  it "includes custom fields" do
    json = described_class.new(category, scope: Guardian.new, root: false).as_json
    expect(json[:custom_fields]).to be_empty

    category.custom_fields["enable_marketplace"] = true
    category.save_custom_fields

    json = described_class.new(category, scope: Guardian.new, root: false).as_json
    expect(json[:custom_fields]).to be_present
  end
end