Add #create_or_find_by to lean on unique constraints (#31989)

Add #create_or_find_by to lean on unique constraints

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+## Rails 6.0.0.alpha (Unreleased) ##
+
+*   Add ActiveRecord::Base.create_or_find_by/! to deal with the SELECT/INSERT race condition in
+    ActiveRecord::Base.find_or_create_by/! by leaning on unique constraints in the database.
+
+    *DHH*
 
 
 Please check [5-2-stable](https://github.com/rails/rails/blob/5-2-stable/activerecord/CHANGELOG.md) for previous changes.

activerecord/lib/active_record/querying.rb

@@ -5,7 +5,7 @@ module ActiveRecord
     delegate :find, :take, :take!, :first, :first!, :last, :last!, :exists?, :any?, :many?, :none?, :one?, to: :all
     delegate :second, :second!, :third, :third!, :fourth, :fourth!, :fifth, :fifth!, :forty_two, :forty_two!, :third_to_last, :third_to_last!, :second_to_last, :second_to_last!, to: :all
     delegate :first_or_create, :first_or_create!, :first_or_initialize, to: :all
-    delegate :find_or_create_by, :find_or_create_by!, :find_or_initialize_by, to: :all
+    delegate :find_or_create_by, :find_or_create_by!, :create_or_find_by, :create_or_find_by!, :find_or_initialize_by, to: :all
     delegate :find_by, :find_by!, to: :all
     delegate :destroy_all, :delete_all, :update_all, to: :all
     delegate :find_each, :find_in_batches, :in_batches, to: :all

activerecord/lib/active_record/relation.rb

@@ -147,18 +147,7 @@ module ActiveRecord
     # or processes there is a race condition between both calls and it could
     # be the case that you end up with two similar records.
     #
-    # Whether that is a problem or not depends on the logic of the
-    # application, but in the particular case in which rows have a UNIQUE
-    # constraint an exception may be raised, just retry:
-    #
-    #  begin
-    #    CreditAccount.transaction(requires_new: true) do
-    #      CreditAccount.find_or_create_by(user_id: user.id)
-    #    end
-    #  rescue ActiveRecord::RecordNotUnique
-    #    retry
-    #  end
-    #
+    # If this might be a problem for your application, please see #create_or_find_by.
     def find_or_create_by(attributes, &block)
       find_by(attributes) || create(attributes, &block)
     end
@@ -170,6 +159,47 @@ module ActiveRecord
       find_by(attributes) || create!(attributes, &block)
     end
 
+    # Attempts to create a record with the given attributes in a table that has a unique constraint
+    # on one or several of its columns. If a row already exists with one or several of these
+    # unique constraints, the exception such an insertion would normally raise is caught,
+    # and the existing record with those attributes is found using #find_by.
+    #
+    # This is similar to #find_or_create_by, but avoids the problem of stale reads between the SELECT
+    # and the INSERT, as that method needs to first query the table, then attempt to insert a row
+    # if none is found.
+    #
+    # There are several drawbacks to #create_or_find_by, though:
+    #
+    # * The underlying table must have the relevant columns defined with unique constraints.
+    # * A unique constraint violation may be triggered by only one, or at least less than all,
+    #   of the given attributes. This means that the subsequent #find_by may fail to find a
+    #   matching record, which will then raise an <tt>ActiveRecord::RecordNotFound</tt> exception,
+    #   rather than a record with the given attributes.
+    # * While we avoid the race condition between SELECT -> INSERT from #find_or_create_by,
+    #   we actually have another race condition between INSERT -> SELECT, which can be triggered
+    #   if a DELETE between those two statements is run by another client. But for most applications,
+    #   that's a significantly less likely condition to hit.
+    # * It relies on exception handling to handle control flow, which may be marginally slower.
+    #
+    # This method will return a record if all given attributes are covered by unique constraints
+    # (unless the INSERT -> DELETE -> SELECT race condition is triggered), but if creation was attempted
+    # and failed due to validation errors it won't be persisted, you get what #create returns in
+    # such situation.
+    def create_or_find_by(attributes, &block)
+      transaction(requires_new: true) { create(attributes, &block) }
+    rescue ActiveRecord::RecordNotUnique
+      find_by!(attributes)
+    end
+
+    # Like #create_or_find_by, but calls
+    # {create!}[rdoc-ref:Persistence::ClassMethods#create!] so an exception
+    # is raised if the created record is invalid.
+    def create_or_find_by!(attributes, &block)
+      transaction(requires_new: true) { create!(attributes, &block) }
+    rescue ActiveRecord::RecordNotUnique
+      find_by!(attributes)
+    end
+
     # Like #find_or_create_by, but calls {new}[rdoc-ref:Core#new]
     # instead of {create}[rdoc-ref:Persistence::ClassMethods#create].
     def find_or_initialize_by(attributes, &block)

activerecord/test/cases/relations_test.rb

@@ -22,6 +22,7 @@ require "models/reader"
 require "models/category"
 require "models/categorization"
 require "models/edge"
+require "models/subscriber"
 
 class RelationTest < ActiveRecord::TestCase
   fixtures :authors, :author_addresses, :topics, :entrants, :developers, :companies, :developers_projects, :accounts, :categories, :categorizations, :categories_posts, :posts, :comments, :tags, :taggings, :cars, :minivans
@@ -1349,6 +1350,34 @@ class RelationTest < ActiveRecord::TestCase
     assert_raises(ActiveRecord::RecordInvalid) { Bird.find_or_create_by!(color: "green") }
   end
 
+  def test_create_or_find_by
+    assert_nil Subscriber.find_by(nick: "bob")
+
+    subscriber = Subscriber.create!(nick: "bob")
+
+    assert_equal subscriber, Subscriber.create_or_find_by(nick: "bob")
+    assert_not_equal subscriber, Subscriber.create_or_find_by(nick: "cat")
+  end
+
+  def test_create_or_find_by_with_non_unique_attributes
+    Subscriber.create!(nick: "bob", name: "the builder")
+
+    assert_raises(ActiveRecord::RecordNotFound) do
+      Subscriber.create_or_find_by(nick: "bob", name: "the cat")
+    end
+  end
+
+  def test_create_or_find_by_within_transaction
+    assert_nil Subscriber.find_by(nick: "bob")
+
+    subscriber = Subscriber.create!(nick: "bob")
+
+    Subscriber.transaction do
+      assert_equal subscriber, Subscriber.create_or_find_by(nick: "bob")
+      assert_not_equal subscriber, Subscriber.create_or_find_by(nick: "cat")
+    end
+  end
+
   def test_find_or_initialize_by
     assert_nil Bird.find_by(name: "bob")