This is something I could definitely use. Ran into this exact issue (needing a deferred unique constraint) implementing a drag-and-drop feature with https://github.com/brendon/acts_as_list. Thank you!

If the Rails team doesn't want to add the new methods, another possible syntax we could use is:

add_index :items, [:position], unique: true

add_index :items, [:position], unique: { deferrable: true }

add_index :items, [:position], unique: { deferrable: :deferred }

Thanks for the information. Let me confirm that brendon/acts_as_list#378 is the issue you are referring to?

The question above is for @sergiopantoja

If the Rails team doesn't want to add the new methods, another possible syntax we could use is:
add_index :items, [:position], unique: { deferrable: true }

@sergiopantoja The syntax of add_index(... , unique: true, deferrable: :true) is confusing and I don't want to support it.
Since UNIQUE INDEX is a restriction on the structure of the index, there is no such thing as a DEFERRABLE unique index. Therefore, CREATE UNIQUE INDEX ~ DEFERRABLE is a syntax error.

The UNIQUE CONSTRAINT added this PR is a unique constraint. And it can define as DEFERRABLE.
When adding a unique constraint implicitly creates an index, but this is to speed up the DB's internal processing.

Let me leave some comments.

• Method name add_unique_constraint

How about add_unique_key because there is add_foreign_key method to create foreign keys?

add_*_key looks consistent to me because these methods create database-level unique/foreign key constraints rather than following add_check_constrainit.

• deferrable: true and deferrable: :deferred are hard to understand to me. Both true and :deferred are truthy values.

• Unique constraints can be created on the same column as the unique index created.

As far as I understand, PostgreSQL allows users to create a unique constraints on top of the unique index
that cannot be deferred. I assume most of the users who need to create deferrable unique constraints must have already created unique index by using add_index with unique: true.

In this case, even if users creates deferrable unique constraint, the existing unique index does not allow users
to violate uniqueness within the transaction. So I think some documentation needs to be updated like remove_index is suggested before creating deferrable unique constraints.

@yahonda Thank you for reviewing.

Method name add_unique_constraint
How about add_unique_key because there is add_foreign_key method to create foreign keys?

add_*_key looks consistent to me because these methods create database-level unique/foreign key constraints rather than following add_check_constrainit.

Okay, I'm going to rename those methods.

deferrable: true and deferrable: :deferred are hard to understand to me. Both true and :deferred are truthy values.

I think the deferrable should be the same option as add_foreign_key, don't you? Adds support for deferrable foreign key constraints in PostgreSQL #41487

Unique constraints can be created on the same column as the unique index created.

As far as I understand, PostgreSQL allows users to create a unique constraints on top of the unique index
that cannot be deferred. I assume most of the users who need to create deferrable unique constraints must have already created unique index by using add_index with unique: true.

In this case, even if users creates deferrable unique constraint, the existing unique index does not allow users
to violate uniqueness within the transaction. So I think some documentation needs to be updated like remove_index is suggested before creating deferrable unique constraints.

👍

@yahonda I fixed some based on your reviewing 😄

• renamed add_unique_constraint to add_unique_key
• Added some documentations.
  • https://github.com/rails/rails/pull/46192/files#diff-353f4f101b1bb04938d761b6e29c88e0f147084e9e276854a77ae5d9a9a3b2a8R46-R47
  • https://github.com/rails/rails/pull/46192/files#diff-0989923401a81fa4208ffc75f8e8622d20353373f9ba38c373d7ff15485b20b4R690-R694

I have one question.

The deferrable: true leaves the default behavior,

Let me confirm this is correct. Adding deferrable: true in the migration adds DEFERRABLE at the end of the SQL statement which allows unique constraints to check deferred.

• Migration

add_unique_key :users, [:name], deferrable: true

• SQL generated by this migration

ALTER TABLE "users" ADD CONSTRAINT uniq_rails_4a6f7b0ccc UNIQUE ("name") DEFERRABLE

Here is my example using psql and raw SQL statements.

• SQL without DEFERRABLE

CREATE TABLE example(
    row integer NOT NULL,
    col integer NOT NULL,
    UNIQUE (row, col)
);

begin;
INSERT INTO example (row, col) VALUES (1,1),(2,2),(3,3);
commit;

SELECT * FROM example;


begin;
UPDATE example SET row = row + 1, col = col + 1; 
# This statement raises `ERROR:  duplicate key value violates unique constraint "example_row_col_key"
# DETAIL:  Key ("row", col)=(2, 2) already exists.` error
commit;

SELECT * FROM example;

• Result without DEFERRABLE

CREATE TABLE
BEGIN
INSERT 0 3
COMMIT
 row | col
-----+-----
   1 |   1
   2 |   2
   3 |   3
(3 rows)

BEGIN
ERROR:  duplicate key value violates unique constraint "example_row_col_key"
DETAIL:  Key ("row", col)=(2, 2) already exists.
ROLLBACK
 row | col
-----+-----
   1 |   1
   2 |   2
   3 |   3
(3 rows)

test46192=#

• SQL with DEFERRABLE

CREATE TABLE example(
    row integer NOT NULL,
    col integer NOT NULL,
    UNIQUE (row, col) DEFERRABLE
);

begin;
INSERT INTO example (row, col) VALUES (1,1),(2,2),(3,3);
commit;

SELECT * FROM example;


begin;
UPDATE example SET row = row + 1, col = col + 1; # This statement should update the rows
commit;

SELECT * FROM example;

• Result with DEFERRABLE

CREATE TABLE
BEGIN
INSERT 0 3
COMMIT
 row | col
-----+-----
   1 |   1
   2 |   2
   3 |   3
(3 rows)

BEGIN
UPDATE 3
COMMIT
 row | col
-----+-----
   2 |   2
   3 |   3
   4 |   4
(3 rows)

test46192=#

My previous question is more like confirming PostgreSQL behavior, so let me answer the question.

I think the deferrable should be the same option as add_foreign_key,
don't you? Adds support for deferrable foreign key constraints in PostgreSQL #41487

Now I understand this API design comes from add_foreign_key. Do you have any other ideas for API design without considering current add_foreign_key? If the new API design is better, we may consider deprecating the current behavior. API compatibility is important. it would be difficult to change the API of add_foreign_key, though. I'd like to know.

@yahonda

Let me confirm this is correct. Adding deferrable: true in the migration adds DEFERRABLE at the end of the SQL statement which allows unique constraints to check deferred.

I didn't know there was a no conflict with bulk update without specifying SET CONSTRAINTS ALL DEFERRED. Thanks for the confirmation.

Now I understand this API design comes from add_foreign_key. Do you have any other ideas for API design without considering current add_foreign_key? If the new API design is better, we may consider deprecating the current behavior. API > compatibility is important. it would be difficult to change the API of add_foreign_key, though. I'd like to know.

I like the current add_foreign_key API.

Hmmm...
If I had to change it, I think it would be possible to get rid of deferrable: true and change it to only allow deferrable: :immediate | :deferred symbols.

If I had to change it, I think it would be possible to get rid of deferrable: true and change it to only allow deferrable: :immediate | :deferred symbols.

If just setting DEFERRABLE without INITIALLY DEFERRED or INITIALLY IMMEDIATE clause
means one of INITIALLY DEFERRED or INITIALLY IMMEDIATE, I think we would remove true value from deferrable: arguments.

Reading PostgreSQL document however, I'm not clear what is an expected behavior only when DEFERRABLE specified.

https://www.postgresql.org/docs/15/sql-set-constraints.html

Upon creation, a constraint is given one of three characteristics: DEFERRABLE INITIALLY DEFERRED, DEFERRABLE INITIALLY IMMEDIATE, or NOT DEFERRABLE. The third class is always IMMEDIATE and is not affected by the SET CONSTRAINTS command. The first two classes start every transaction in the indicated mode, but their behavior can be changed within a transaction by SET CONSTRAINTS.

Reading PostgreSQL document however, I'm not clear what is an expected behavior only when DEFERRABLE specified.

Investigated how DEFERRABLE without any option works and found it means DEFERRABLE INITIALLY IMMEDIATE.
This behavior is documented and consistent between PostgreSQL 9.3, the oldest version supported by the Rails main branch, and the latest PostgreSQL 15.

https://www.postgresql.org/docs/9.3/sql-createtable.html

If a constraint is deferrable, this clause specifies the default time to check the constraint. If the constraint is INITIALLY IMMEDIATE, it is checked after each statement. This is the default. If the constraint is INITIALLY DEFERRED, it is checked only at the end of the transaction. The constraint check time can be altered with the SET CONSTRAINTS command.

https://www.postgresql.org/docs/15/sql-createtable.html

If a constraint is deferrable, this clause specifies the default time to check the constraint. If the constraint is INITIALLY IMMEDIATE, it is checked after each statement. This is the default. If the constraint is INITIALLY DEFERRED, it is checked only at the end of the transaction. The constraint check time can be altered with the SET CONSTRAINTS command.

Here are my test details. https://gist.github.com/yahonda/1adcf1a776a385b8ec47c9cdf5dcf08b

Based on how PostgreSQL works, investigated at #46192 (comment)

I'd like to propose that deferrable: only accepts :immediate or :deferred.

• Example

add_unique_key :items, [:position], deferrable: :immediate

add_unique_key :items, [:position], deferrable: :deferred

If deferrable: true or other invalid option name is given, it should raise an error or suggest the correct argument name.

@yahonda Thank you for your detailed investigation 🙏

I have updated some fixes and rebased 😄
Since force-push and rebase make it difficult to display changes, I also provide a URL to view the diff.
diff

The changes are as follows

• CHANGELOG
  • Now breaks lines after a certain length.
  • Corrected explanation of option values and default behavior.
• The variable unique_constraint is renamed to unique_key when changing from add_unique_constraint to add_unique_key, which was forgotten to be renamed.
• Changed deferrable: to only accept :immediate and :deferred.
  • It no longer shares the conversion logic with add_foreign_key.
    • NOTE: To unify the behavior of deferrable: true for add_foreign_key with add_unique_key, we need to deprecate deferred: true for add_foreign_key in another PR. I will try to fix this in the next PR.
• Fixed a bug in the condition that throws an ActiveRecord::IrreversibleMigration exception when remove_unique_key cannot be rolled back.
• Added some tests

Update to anyone landing here from Google or this post, this was renamed back to add_unique_constraint (t.unique_constraint also works)

@searls FYI : here is the changelog link locked with SHA by pressing "Y" on the page.

This link will work "forever"