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

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!

I guess I have to code something that will automatically delete the comments? — Thomas Cailhol, Jun 21 '17 at 12:28
It's fixed guys, I didn't have the dependent: :destroy in my post model thanks a lot — Thomas Cailhol, Jun 21 '17 at 12:47

Ursus · Accepted Answer · 2019-10-11T07:24:39.950

7

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

edited Oct 11 '19 at 07:24

answered Jun 21 '17 at 12:30

Ursus

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Awesome it worked perfectly thanks so much! – Thomas Cailhol Jun 21 '17 at 12:44

hgsongra · Answer 2 · 2017-06-21T12:40:46.397

1

Update the below line in your Post model as below

has_many :comments, dependent: :destroy

You have to mention dependent: :destroy in your Post model. So when any post get to deleted by post.destroy, It deletes the all dependent records of Comment model.

Hope this will resolve your issue

edited Jun 21 '17 at 12:40

answered Jun 21 '17 at 12:30

hgsongra

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It fixed my problem. Thanks a lot! – Thomas Cailhol Jun 21 '17 at 12:45
Welcome :) Keep coding – hgsongra Jun 21 '17 at 12:50

score 1 · Answer 3 · answered Jun 21 '17 at 12:31

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

Worked perfectly!! Thanks! – Thomas Cailhol Jun 21 '17 at 12:44 — Thomas Cailhol, Jun 21 '17 at 12:44

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

3 Answers3