ActiveRecord::HasManyThroughAssociationNotFoundError in UserController#welcome
You need to define the join model as a separate association when using has_many :through:
class Post < ActiveRecord::Base has_many :user_posts has_many :users, :through => :user_postsendclass User < ActiveRecord::Base has_many :user_posts has_many :posts, :through => :user_postsendclass UserPost < ActiveRecord::Base belongs_to :user # foreign_key is user_id belongs_to :post # foreign_key is post_idendThis works best when you need to keep data that pertains to the join model itself, or if you want to perform validations on the join separate from the other two models.
If you just want a simple join table, it's easier to use the old HABTM syntax:
class User < ActiveRecord::Base has_and_belongs_to_many :postsendclass Post < ActiveRecord::Base has_and_belongs_to_many :usersend