Data Modeling 3 Way Table has_many association

问题

I am attempting to build a table to handle both the location and category a certain campaign has been set to with the following model associations:

class Campaign < ActiveRecord::Base

    has_many :campaign_category_metro_bids, dependent: :destroy
    has_many :metros,     through: :campaign_category_metro_bids
    has_many :categories, through: :campaign_category_metro_bids

end

class Metro < ActiveRecord::Base

    has_many :campaign_category_metro_bids
    has_many :campaigns,  through: :campaign_category_metro_bids
    has_many :categories, through: :campaign_category_metro_bids

end

class Category < ActiveRecord::Base

    has_many :campaign_category_metro_bids
    has_many :campaigns,  through: :campaign_category_metro_bids
    has_many :metros,     through: :campaign_category_metro_bids

end

class CampaignCategoryMetroBid < ActiveRecord::Base
    belongs_to :campaign
    belongs_to :category
    belongs_to :metro
end

When attempting to create a campaign for selecting two different cities and categories the result is NULL for the id of one of the paramters as:

Campaign creation code:

def new
    if signed_in?
        # create new campaign
        @user = User.find(params[:id])
        @campaign = @user.campaigns.new
    else
        redirect_to signin_path
    end
end

def create
    @campaign = User.find(params["campaign"]["user_id"]).campaigns.build(campaign_params)

    if @campaign.save
        flash[:success] = "Campaign created!"
        redirect_to current_user
    else
        render 'new'
    end
end

UPDATED The view to create the campaign uses two separate collection_select for Category and Metro as:

        <%= f.collection_select :category_ids, Category.all, :id, :display_category, {}, {multiple: true} %>

and

    <%= f.collection_select :metro_ids, Metro.all, :id, :full_name, {}, {multiple: true} %>

campaigns_params:

    def campaign_params
        params.require(:campaign).permit(:name, :campaign_category_metro_bid_id,
                                         :metro_ids => [], :category_ids => [])
    end

Is there a better way to allow for the creation of a 3 table relation as I am attempting? or a way to link the Category and Metro models at selection so that the resultant table is something like below upon campaign creation:

回答1:

I think the problem may be the multi select that you have on categories and metros. You're essentially trying to fit multiple foreign_keys for the same reference into a single row record. If category ID and metro ID are both defined as integers, you would need to create multiple records in order to be able to save this.

You would need to add some logic to see if your selection params have a length of > 1 and based on that you'll need to create and save a new row. The logic would look something like this

params[:category_ids].each do |category|
  params[:metro_ids].each do |metro|
    @user.campaign.create(category_id: category, metro_id:metro) #any other params would go here too
  end
end

This would essentially loop through your multi-select to create a new record for each combination.

来源：https://stackoverflow.com/questions/26028291/data-modeling-3-way-table-has-many-association