Ensure #second (and others) acts like #first AR finder #13757

terracatta · 2014-01-18T23:39:52Z

This PR addresses issue #13743 where the ordinal ActiveSupport Array extentions were not assuming a default order like first was.

As suggested by @dhh this commit brings the famous ordinal Array instance methods defined
in ActiveSupport into ActiveRecord as fully-fledged finders.

These finders ensure a default ascending order of the table's primary key, and utilize the OFFSET SQL verb to locate the user's desired record. If an offset is defined in the query, calling #second adds
to the offset to get the actual desired record.

Example:

    User.all.second

    # Before
    # => 'SELECT  "users".* FROM "users"'

    # After
    # => SELECT  "users".* FROM "users"   ORDER BY "users"."id" ASC LIMIT 1 OFFSET 1'

    User.offset(3).second

    # Before
    # => 'SELECT "users".* FROM "users"  LIMIT -1 OFFSET 3' # sqlite3 gem
    # => 'SELECT "users".* FROM "users"  OFFSET 3' # pg gem
    # => 'SELECT `users`.* FROM `users`  LIMIT 18446744073709551615 OFFSET 3' # mysql2 gem"'

    # After
    # => SELECT  "users".* FROM "users"   ORDER BY "users"."id" ASC LIMIT 1 OFFSET 4'

dhh · 2014-01-19T01:06:44Z

Looking good to me! 👍

josevalim · 2014-01-19T13:46:25Z

activerecord/lib/active_record/relation/finder_methods.rb

-    def find_first_with_limit(limit)
+    def find_nth_with_limit(ordinal, limit)
+      offset = case ordinal
+      when :first


This function should not have knowledge of :first, :second and friends. We should probably just pass the offset_value as argument.

terracatta · 2014-01-19T20:21:56Z

@josevalim Ok, fixed that up for you! Any other concerns?

senny · 2014-01-20T11:13:12Z

activerecord/CHANGELOG.md

+    The famous ordinal Array instance methods defined in ActiveSupport
+    (`first`, `second`, `third`, `fourth`, and `fifth`) are now available as
+    full-fledged finders in ActiveRecord. The biggest benefit of this is ordering
+    of the records returned now defaults to the table's primary key in ascending order.


Can you add:

Fixes #13743.

Added it after the examples! Thanks for the heads up!

senny · 2014-01-20T11:18:02Z

@terracatta this looks good. Can you squash your commits?

carlosantoniodasilva · 2014-01-20T11:33:14Z

activerecord/lib/active_record/relation/finder_methods.rb

      if loaded?
        @records.first
      else
-        @first ||= find_first_with_limit(1).first
+        @nth ||= find_nth_with_limit(offset, 1).first


I have a question: how problematic can this cached variable be now that we are using it for different "finds"?

Very problematic. It will cache only the first call.

Ok so I removed the caching here, but obviously that isn't the right way to go as that will cause multiple queries to fire unnecessarily when calling .first a bunch of times. I think the only option here is cache based on offset being passed into the private method.

Any other suggestions?

ok I just went with that

terracatta · 2014-01-20T14:09:16Z

@rafaelfranca @carlosantoniodasilva @senny, commits are squashed, updated the changelog, fixed the caching issue by dynamically setting and manually memoizing the instance variable based on the offset passed in. Tested it again on several personal rails projects and the test suite looks good!

Any other feedback?

rafaelfranca · 2014-01-20T15:05:58Z

activerecord/lib/active_record/relation/finder_methods.rb

      if loaded?
        @records.first
      else
-        @first ||= find_first_with_limit(1).first
+        unless instance_variable_defined?("@offset_#{offset||0}")


It is better to initialize a empty hash in the initialize of Relation and use the offset as the key to memoize it.

Since we are adding an internal variable to cache the value we have to update reset method

rails/activerecord/lib/active_record/relation.rb

Line 497 in 080fc9c

def reset

Great suggestion, I'll implement it ASAP

terracatta · 2014-01-20T21:28:18Z

@rafaelfranca, I made the requested changes, but couldn't reproduce your issue or create a failing test case (still trying), can you try again with the latest code and see if you still get a negative limit?

This commit bring the famous ordinal Array instance methods defined in ActiveSupport into ActiveRecord as fully-fledged finders. These finders ensure a default ascending order of the table's primary key, and utilize the OFFSET SQL verb to locate the user's desired record. If an offset is defined in the query, calling #second adds to the offset to get the actual desired record. Fixes #13743.

terracatta · 2014-01-20T22:01:00Z

@rafaelfranca Ok great, added better before examples for pg, sqlite3, and mysql2 gems. I also added some extra test cases for the offset functionality just in case!

Anything else? Sorry for all the back and forth!

rafaelfranca · 2014-01-20T22:43:20Z

Looks good. Thank you so much for working on this.

Ensure #second (and others) acts like #first AR finder

rafaelfranca · 2014-01-20T22:48:39Z

Merged ❤️

carlosantoniodasilva · 2014-01-21T11:56:48Z

activerecord/lib/active_record/relation/finder_methods.rb

@@ -364,19 +444,19 @@ def find_take
      end
    end

-    def find_first
+    def find_nth(offset)
      if loaded?
        @records.first


This is apparently not going to work with already loaded associations. Just testing around I got two different issues:

# consider the models `Author` has many `Post`s ## 1st scenario - raises exception >> carlos.posts.reload.first Post Load (0.1ms) SELECT "posts".* FROM "posts" WHERE "posts"."author_id" = ? [["author_id", 1]] => #<Post id: 1, author_id: 1, title: "ZOMG", body: nil, created_at: "2014-01-21 11:44:22", updated_at: "2014-01-21 11:44:22"> >> carlos.posts.reload.second Post Load (0.1ms) SELECT "posts".* FROM "posts" WHERE "posts"."author_id" = ? [["author_id", 1]] NoMethodError: undefined method 'first' for nil:NilClass from....rails/rails/activerecord/lib/active_record/relation/finder_methods.rb:449:in 'find_nth' ## 2nd scenario - returns the same record >> carlos.posts.first => #<Post id: 1, author_id: 1, title: "ZOMG", body: nil, created_at: "2014-01-21 11:44:22", updated_at: "2014-01-21 11:44:22"> >> carlos.posts.second => #<Post id: 1, author_id: 1, title: "ZOMG", body: nil, created_at: "2014-01-21 11:44:22", updated_at: "2014-01-21 11:44:22"> >> carlos.posts.third => #<Post id: 1, author_id: 1, title: "ZOMG", body: nil, created_at: "2014-01-21 11:44:22", updated_at: "2014-01-21 11:44:22">

@terracatta can you take a look please? It'd be good to provide some tests for already loaded associations with some of these methods. Thanks.

Absolutely! Thanks for testing!

josevalim reviewed Jan 19, 2014
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senny reviewed Jan 20, 2014
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carlosantoniodasilva reviewed Jan 20, 2014
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rafaelfranca reviewed Jan 20, 2014
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rafaelfranca added a commit that referenced this pull request Jan 20, 2014

Merge pull request #13757 from terracatta/master

bc62508

Ensure #second (and others) acts like #first AR finder

rafaelfranca merged commit cafe31a into rails:master Jan 20, 2014

carlosantoniodasilva reviewed Jan 21, 2014
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terracatta mentioned this pull request Jan 21, 2014

New finder methods (#second, #third) etc do not work correctly on associations #13783

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Ensure #second (and others) acts like #first AR finder #13757

Ensure #second (and others) acts like #first AR finder #13757

terracatta commented Jan 18, 2014

dhh commented Jan 19, 2014

josevalim Jan 19, 2014

terracatta commented Jan 19, 2014

senny Jan 20, 2014

terracatta Jan 20, 2014

senny commented Jan 20, 2014

carlosantoniodasilva Jan 20, 2014

rafaelfranca Jan 20, 2014

terracatta Jan 20, 2014

terracatta Jan 20, 2014

terracatta commented Jan 20, 2014

rafaelfranca Jan 20, 2014

terracatta Jan 20, 2014

terracatta commented Jan 20, 2014

terracatta commented Jan 20, 2014

rafaelfranca commented Jan 20, 2014

rafaelfranca commented Jan 20, 2014

carlosantoniodasilva Jan 21, 2014

terracatta Jan 21, 2014

Ensure #second (and others) acts like #first AR finder #13757

Ensure #second (and others) acts like #first AR finder #13757

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terracatta commented Jan 18, 2014

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terracatta commented Jan 20, 2014

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