Rails 6 如何将 model 的多个字段作为对另一个表的引用

Question

I'm currently creating a Rails app using rails 6. I have model named user_teams and a model named players. What I want to do is have multiple fields in the user_teams model such as player_one, player_two...player_six that are each an individual reference to a player.

I have implemented this but in a slightly janky way. I have the six fields in my user_team model and each is an integer of the id associated with the chosen player. I feel like there is a much better way to do this but I can't figure out how. Any help would be greatly appreciated!

Also once I have the references set up properly, how can I validate each player reference so that duplicate players can't be in the same user_team?

Example UserTeam player_one: 1 player_two: 1

Would not be allowed but

UserTeam player_one: 1 player_two: 2

Would be allowed?

Answer 1

This is a standard many to many association and what you want is a join table that joins players and teams:

class Player < ApplicationRecord
  has_many :positions
  has_many :user_teams, through: :positions
end

class UserTeam < ApplicationRecord
  has_many :positions
  has_many :players, through: :positions
end

# generate the model and migration with:
# rails g model position player:belongs_to user_team:belongs_to
class Position < ApplicationRecord
  belongs_to :player
  belongs_to :user_team
end

Another way to set this up would be with has_and_belongs_to_many but as there is no join model there is no way to access any additional columns on the join table and you cannot join it directly.

You can validate the uniqueness of a player on a team by using validates_uniqueness with the scope option:

class Position < ApplicationRecord
  belongs_to :player
  belongs_to :user_team
  validates :player_id, uniqueness: { scope: :user_team_id }
end 

This will prevent most duplicates but is prone to race conditions which could lead to duplicate data being inserted so it should be complimented with a unique index that enforces the constraint on the database level:

class CreatePositions < ActiveRecord::Migration[6.0]
  def change
    create_table :positions do |t|
      t.belongs_to :user_team, null: false, foreign_key: true
      t.belongs_to :player, null: false, foreign_key: true
      t.timestamps
    end
    add_index :positions, [:user_team_id, :player_id], unique: true
  end
end

Answer 2

UserTeam model should have field name & Player model should have field user_team_id

model/UserTeam.rb

class UserTeam < ApplicationRecord
  has_many :players
end

model/Player.rb

class Player < ApplicationRecord
  belongs_to :user_team
end