Create a record with a multiselect through a many-to-many relationship

class Forest::Card
    include ForestLiana::Collection
    collection :Card

    action 'Create Card',
        type: 'global',
        fields: [{
            field: "name",
            type: "String",
            isRequired: true,
        },
        {
            field: "user",
            type: "Number",
            reference: "User.id",
            isRequired: true,
        },
        {
            field: "company",
            type: "Number",
            reference: "Company.id",
            isRequired: true,
        },
        {
            field: "vendor",
            type: "Number",
            reference: "Vendor.id",
            isRequired: true,
        },
        {
            field: "categories",
            type: ['Enum'],
        }
        ],
        :hooks => {
            :load => -> (context) {
                categories = context[:fields].find{|field| field[:field] == 'categories'}
                categories[:enums] = ExpenseCategory.all.pluck(:title)
                return context[:fields]
            }
        }
end

Rails.application.routes.draw do
  ...
  namespace :forest do
    post '/actions/create-card' => 'cards#create_card'
  end
  mount ForestLiana::Engine => '/forest'
end

class Forest::CardsController < ForestLiana::SmartActionsController
    def create_card
        attrs = params.dig('data', 'attributes', 'values')
        categories_attrs = attrs['categories'];
        attrs = { name: attrs['name'], user_id: attrs['user'], company_id: attrs['company'], vendor_id: attrs['vendor'] };

        card = Card.create(attrs)
        categories_attrs.each do|category|
            expense_category = ExpenseCategory.find_by(title: category)
            card_expense_category = CardExpenseCategory.create(card_id: card.id, expense_category_id: expense_category.id)
        end

        render json: { success: 'Your card has been created.' }
    end
end