update or delete on table "x" violates foreign key constraint "fk_rails_5a7b40847a" on table "x" [duplicate]

Question

I am building a rails app (5.1.1) that has the equivalent of posts and comments linked to them.

Everything seems to be working just fine but when I try to delete a post that has comments I get this error:

PG::ForeignKeyViolation: ERROR:  update or delete on table "posts" violates foreign key constraint "fk_rails_5a7b40847a" on table "comments"
DETAIL:  Key (id)=(3) is still referenced from table "comments".
: DELETE FROM "posts" WHERE "prototypes"."id" = $1"

The error seems pretty straight forward but I'm really new to rails and postgresql so I'm looking for some help on that!

Answer 1

It depends what you want to do with the post's comments on its deletion. In case you want to delete them on cascade, for example:

post.rb

has_many :comments, dependent: :destroy

Answer 2

It's because you have a constraint in your database.
I presume that the foreign key post_id in table comments must exist in the associated table posts.
That is certainly because you specify the foreign key relation in the migration with rails g model Comment post:references.

You have to specify what to do with associated models on deletion :

class Post < ActiveRecord::Base
  has_many :comments, dependent: :destroy # destroy associated comments
end

Then call method post.destroy instead of post.delete on your record

See has_many for other options