why PG::UniqueViolation: ERROR: duplicate key value violates unique constraint? - ruby-on-rails

Question

I have a model Post and each time a post is created I want a new instance of Moderation to be created at the same time.
So in post.rb I use the callback after_save :create_moderation
Then write a private method :
...
include Reportable
after_save :create_moderation
private
def create_moderation
self.create_moderation!(blog: Blog.first)
end
But when a proposal is created I get this error :
PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "moderations_reportable" DETAIL: Key (reportable_type, reportable_id)=(Post, 25) already exists. : INSERT INTO "moderations" ("blog_id", "reportable_type", "reportable_id", "created_at", "updated_at", "blog_type") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"
In reportable.rb I have :
has_one :moderation, as: :reportable, foreign_key: "reportable_id", foreign_type: "reportable_type", class_name: "Moderation"
Then few other methods for reportable object.
Note that this issue doesn't happen when I run the create method in console.
EDIT
create_table "moderations", id: :serial, force: :cascade do |t|
t.string "reportable_type", null: false
t.string "reportable_id", null: false
t.integer "blog_id", null: false
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.string "blog_type", null: false
t.string "upstream_moderation", default: "unmoderate"
t.index ["blog_id", "blog_type"], name: "moderations_blog"
t.index ["reportable_type", "reportable_id"], name: "moderations_reportable", unique: true
end
create_table "posts", id: :serial, force: :cascade do |t|
t.text "title", null: false
t.text "body", null: false
t.integer "feature_id", null: false
t.integer "author_id"
t.integer "scope_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "post_votes_count", default: 0, null: false
t.index ["body"], name: "post_body_search"
t.index ["created_at"], name: "index_posts_on_created_at"
t.index ["author_id"], name: "index_posts_on_author_id"
t.index ["feature_id"], name: "index_posts_on_feature_id"
t.index ["proposal_votes_count"], name: "index_posts_on_post_votes_count"
t.index ["title"], name: "post_title_search"
end

Answer 1

To fix the issue, we have to tell ActiveRecord to look at the sequence of the table:
ActiveRecord::Base.connection.reset_pk_sequence!('table_name')
Now ActiveRecord should have the correct sequence value, and should be able to assign new id's properly.
To resolve error
PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "moderations_reportable" DETAIL: Key (reportable_type, reportable_id)=(Post, 25) already exists. : INSERT INTO "moderations" ("blog_id", "reportable_type", "reportable_id", "created_at", "updated_at", "blog_type") VALUES ($1, $2, $3, $4, $5, $6) RETURNING "id"
As error occurred on 'moderations' table.
Run the following from rails console fix
ActiveRecord::Base.connection.reset_pk_sequence!('moderations')
Thank you

Answer 2

Fix pkey sequences for all database:
ActiveRecord::Base.connection.tables.each do |table_name|
ActiveRecord::Base.connection.reset_pk_sequence!(table_name)
end

Answer 3

Looks like you've added a unique index to your database:
t.index ["reportable_type", "reportable_id"], name: "moderations_reportable", unique: true
With a unique index you will only be able to have one record with the same reportable_type and reportable_id. It's likely that you're trying to create a moderation for a reportable that already has a moderation.

Answer 4

ONLY FOR THOSE WHO ARE SEEING THIS WHILE RUNNING TESTS
One possible reason is, you have some migrations that create new data-records and you ran one of the following recently.
rake db:migrate RAILS_ENV=test
or
rake db:migrate:reset RAILS_ENV=test
Solution
You need to clean/wipeout your test-DB. Run this
rake db:reset RAILS_ENV=test
This will wipeout and generate schema from schema.rb.

Answer 5

I think this can also resolve your problem, Add this in your staging or preprod console ( I am using Heroku for this so I added in my heroku rails console)
connection = ActiveRecord::Base.connection
connection.execute("SELECT setval(pg_get_serial_sequence('moderations', 'id'), MAX(id)) FROM moderations;")
Check out this blog for more info and how this is been generated