diff options
Diffstat (limited to 'activerecord')
| -rw-r--r-- | activerecord/CHANGELOG.md | 6 | ||||
| -rw-r--r-- | activerecord/lib/active_record/querying.rb | 2 | ||||
| -rw-r--r-- | activerecord/lib/active_record/relation.rb | 54 | ||||
| -rw-r--r-- | activerecord/test/cases/relations_test.rb | 29 |
4 files changed, 78 insertions, 13 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md index d73c0859ee..bbb0ec575e 100644 --- a/activerecord/CHANGELOG.md +++ b/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. diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb index 3996d5661f..7c8f794910 100644 --- a/activerecord/lib/active_record/querying.rb +++ b/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 diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb index 8e108e4be3..736173ae1b 100644 --- a/activerecord/lib/active_record/relation.rb +++ b/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) diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb index bc7c54dbe0..18e0bed5b6 100644 --- a/activerecord/test/cases/relations_test.rb +++ b/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") |