Rails idiom to avoid duplicates in has_many :through

Question

I have a standard many-to-many relationship between users and roles in my Rails app:

class User < ActiveRecord::Base
  has_many :user_roles
  has_many :ro         


        

Answer 1

Use Array's |= Join Method.

You can use Array's |= join method to add an element to the Array, unless it is already present. Just make sure you wrap the element in an Array.

role                  #=> #<Role id: 1, name: "1">

user.roles            #=> []

user.roles |= [role]  #=> [#<Role id: 1, name: "1">]

user.roles |= [role]  #=> [#<Role id: 1, name: "1">]

Can also be used for adding multiple elements that may or may not already be present:

role1                         #=> #<Role id: 1, name: "1">
role2                         #=> #<Role id: 2, name: "2">

user.roles                    #=> [#<Role id: 1, name: "1">]

user.roles |= [role1, role2]  #=> [#<Role id: 1, name: "1">, #<Role id: 2, name: "2">]

user.roles |= [role1, role2]  #=> [#<Role id: 1, name: "1">, #<Role id: 2, name: "2">]

Found this technique on this StackOverflow answer.

Answer 2

I ran into this today and ended up using #replace, which "will perform a diff and delete/add only records that have changed".

Therefore, you need to pass the union of the existing roles (so they don't get deleted) and your new role(s):

new_roles = [role]
user.roles.replace(user.roles | new_roles)

It's important to note that both this answer and the accepted one are loading the associated roles objects into memory in order to perform the Array diff (-) and union (|). This could lead to performance issues if you're dealing with a large number of associated records.

If that's a concern, you may want to look into options that check for existence via queries first, or use an INSERT ON DUPLICATE KEY UPDATE (mysql) type query for inserting.