Add Relation#find_or_create_by and friends

This is similar to #first_or_create, but slightly different and a nicer
API. See the CHANGELOG/docs in the commit.

Fixes #7853

5 files changed, 133 insertions, 22 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 186c7bf244..1edcd7cfc8 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,35 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Add `find_or_create_by`, `find_or_create_by!` and
+    `find_or_initialize_by` methods to `Relation`.
+
+    These are similar to the `first_or_create` family of methods, but
+    the behaviour when a record is created is slightly different:
+
+        User.where(first_name: 'Penélope').first_or_create
+
+    will execute:
+
+        User.where(first_name: 'Penélope').create
+
+    Causing all the `create` callbacks to execute within the context of
+    the scope. This could affect queries that occur within callbacks.
+
+        User.find_or_create_by(first_name: 'Penélope')
+
+    will execute:
+
+        User.create(first_name: 'Penélope')
+
+    Which obviously does not affect the scoping of queries within
+    callbacks.
+
+    The `find_or_create_by` version also reads better, frankly. But note
+    that it does not allow attributes to be specified for the `create`
+    that are not included in the `find_by`.
+
+    *Jon Leighton*
+
 *   Fix bug with presence validation of associations. Would incorrectly add duplicated errors 
     when the association was blank. Bug introduced in 1fab518c6a75dac5773654646eb724a59741bc13.
 
@@ -607,9 +637,9 @@
       * `find_or_initialize_by_...` can be rewritten using
         `where(...).first_or_initialize`
       * `find_or_create_by_...` can be rewritten using
-        `where(...).first_or_create`
+        `find_or_create_by(...)` or where(...).first_or_create`
       * `find_or_create_by_...!` can be rewritten using
-        `where(...).first_or_create!`
+        `find_or_create_by!(...) or `where(...).first_or_create!`
 
     The implementation of the deprecated dynamic finders has been moved
     to the `activerecord-deprecated_finders` gem. See below for details.
diff --git a/activerecord/lib/active_record/querying.rb b/activerecord/lib/active_record/querying.rb
index 13e09eda53..45f6a78428 100644
--- a/activerecord/lib/active_record/querying.rb
+++ b/activerecord/lib/active_record/querying.rb
@@ -3,6 +3,7 @@ module ActiveRecord
   module Querying
     delegate :find, :take, :take!, :first, :first!, :last, :last!, :exists?, :any?, :many?, :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_by, :find_by!, :to => :all
     delegate :destroy, :destroy_all, :delete, :delete_all, :update, :update_all, :to => :all
     delegate :find_each, :find_in_batches, :to => :all
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index ecce7c703b..d106fceca2 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -133,6 +133,10 @@ module ActiveRecord
     #
     # Expects arguments in the same format as +Base.create+.
     #
+    # Note that the <tt>create</tt> will execute within the context of this scope, and that may for example
+    # affect the result of queries within callbacks. If you don't want this, use the <tt>find_or_create_by</tt>
+    # method.
+    #
     # ==== Examples
     #   # Find the first user named Penélope or create a new one.
     #   User.where(:first_name => 'Penélope').first_or_create
@@ -171,6 +175,28 @@ module ActiveRecord
       first || new(attributes, &block)
     end
 
+    # Finds the first record with the given attributes, or creates it if one does not exist.
+    #
+    # See also <tt>first_or_create</tt>.
+    #
+    # ==== Examples
+    #   # Find the first user named Penélope or create a new one.
+    #   User.find_or_create_by(first_name: 'Penélope')
+    #   # => <User id: 1, first_name: 'Penélope', last_name: nil>
+    def find_or_create_by(attributes, &block)
+      find_by(attributes) || create(attributes, &block)
+    end
+
+    # Like <tt>find_or_create_by</tt>, but calls <tt>create!</tt> so an exception is raised if the created record is invalid.
+    def find_or_create_by!(attributes, &block)
+      find_by(attributes) || create!(attributes, &block)
+    end
+
+    # Like <tt>find_or_create_by</tt>, but calls <tt>new</tt> instead of <tt>create</tt>.
+    def find_or_initialize_by(attributes, &block)
+      find_by(attributes) || new(attributes, &block)
+    end
+
     # Runs EXPLAIN on the query or queries triggered by this relation and
     # returns the result as a string. The string is formatted imitating the
     # ones printed by the database shell.
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index fdbc0a3fdb..7504da01d5 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1058,6 +1058,29 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal 'parrot', parrot.name
   end
 
+  def test_find_or_create_by
+    assert_nil Bird.find_by(name: 'bob')
+
+    bird = Bird.find_or_create_by(name: 'bob')
+    assert bird.persisted?
+
+    assert_equal bird, Bird.find_or_create_by(name: 'bob')
+  end
+
+  def test_find_or_create_by!
+    assert_raises(ActiveRecord::RecordInvalid) { Bird.find_or_create_by!(color: 'green') }
+  end
+
+  def test_find_or_initialize_by
+    assert_nil Bird.find_by(name: 'bob')
+
+    bird = Bird.find_or_initialize_by(name: 'bob')
+    assert bird.new_record?
+    bird.save!
+
+    assert_equal bird, Bird.find_or_initialize_by(name: 'bob')
+  end
+
   def test_explicit_create_scope
     hens = Bird.where(:name => 'hen')
     assert_equal 'hen', hens.new.name
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md
index 66e6390f67..5e6799e786 100644
--- a/guides/source/active_record_querying.md
+++ b/guides/source/active_record_querying.md
@@ -1225,17 +1225,17 @@ WARNING: Up to and including Rails 3.1, when the number of arguments passed to a
 Find or build a new object
 --------------------------
 
-It's common that you need to find a record or create it if it doesn't exist. You can do that with the `first_or_create` and `first_or_create!` methods.
+It's common that you need to find a record or create it if it doesn't exist. You can do that with the `find_or_create_by` and `find_or_create_by!` methods.
 
-### `first_or_create`
+### `find_or_create_by` and `first_or_create`
 
-The `first_or_create` method checks whether `first` returns `nil` or not. If it does return `nil`, then `create` is called. This is very powerful when coupled with the `where` method. Let's see an example.
+The `find_or_create_by` method checks whether a record with the attributes exists. If it doesn't, then `create` is called. Let's see an example.
 
-Suppose you want to find a client named 'Andy', and if there's none, create one and additionally set his `locked` attribute to false. You can do so by running:
+Suppose you want to find a client named 'Andy', and if there's none, create one. You can do so by running:
 
 ```ruby
-Client.where(:first_name => 'Andy').first_or_create(:locked => false)
-# => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: false, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
+Client.find_or_create_by(first_name: 'Andy')
+# => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
 ```
 
 The SQL generated by this method looks like this:
@@ -1243,27 +1243,50 @@ The SQL generated by this method looks like this:
 ```sql
 SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1
 BEGIN
-INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 0, NULL, '2011-08-30 05:22:57')
+INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 1, NULL, '2011-08-30 05:22:57')
 COMMIT
 ```
 
-`first_or_create` returns either the record that already exists or the new record. In our case, we didn't already have a client named Andy so the record is created and returned.
+`find_or_create_by` returns either the record that already exists or the new record. In our case, we didn't already have a client named Andy so the record is created and returned.
 
 The new record might not be saved to the database; that depends on whether validations passed or not (just like `create`).
 
-It's also worth noting that `first_or_create` takes into account the arguments of the `where` method. In the example above we didn't explicitly pass a `:first_name => 'Andy'` argument to `first_or_create`. However, that was used when creating the new record because it was already passed before to the `where` method.
+Suppose we want to set the 'locked' attribute to true, if we're
+creating a new record, but we don't want to include it in the query. So
+we want to find the client named "Andy", or if that client doesn't
+exist, create a client named "Andy" which is not locked.
 
-You can do the same with the `find_or_create_by` method:
+We can achive this in two ways. The first is passing a block to the
+`find_or_create_by` method:
 
 ```ruby
-Client.find_or_create_by_first_name(:first_name => "Andy", :locked => false)
+Client.find_or_create_by(first_name: 'Andy') do |c|
+  c.locked = false
+end
+```
+
+The block will only be executed if the client is being created. The
+second time we run this code, the block will be ignored.
+
+The second way is using the `first_or_create` method:
+
+```ruby
+Client.where(first_name: 'Andy').first_or_create(locked: false)
 ```
 
-This method still works, but it's encouraged to use `first_or_create` because it's more explicit on which arguments are used to _find_ the record and which are used to _create_, resulting in less confusion overall.
+In this version, we are building a scope to search for Andy, and getting
+the first record if it existed, or else creating it with `locked:
+false`.
 
-### `first_or_create!`
+Note that these two are slightly different. In the second version, the
+scope that we build will affect any other queries that may happens while
+creating the record. For example, if we had a callback that ran
+another query, that would execute within the `Client.where(first_name:
+'Andy')` scope.
 
-You can also use `first_or_create!` to raise an exception if the new record is invalid. Validations are not covered on this guide, but let's assume for a moment that you temporarily add
+### `find_or_create_by!` and `first_or_create!`
+
+You can also use `find_or_create_by!` to raise an exception if the new record is invalid. Validations are not covered on this guide, but let's assume for a moment that you temporarily add
 
 ```ruby
 validates :orders_count, :presence => true
@@ -1272,19 +1295,24 @@ validates :orders_count, :presence => true
 to your `Client` model. If you try to create a new `Client` without passing an `orders_count`, the record will be invalid and an exception will be raised:
 
 ```ruby
-Client.where(:first_name => 'Andy').first_or_create!(:locked => false)
+Client.find_or_create_by!(first_name: 'Andy')
 # => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank
 ```
 
-As with `first_or_create` there is a `find_or_create_by!` method but the `first_or_create!` method is preferred for clarity.
+There is also a `first_or_create!` method which does a similar thing for
+`first_or_create`.
 
-### `first_or_initialize`
+### `find_or_initialize_by` and `first_or_initialize`
 
-The `first_or_initialize` method will work just like `first_or_create` but it will not call `create` but `new`. This means that a new model instance will be created in memory but won't be saved to the database. Continuing with the `first_or_create` example, we now want the client named 'Nick':
+The `find_or_initialize_by` method will work just like
+`find_or_create_by` but it will call `new` instead of `create`. This
+means that a new model instance will be created in memory but won't be
+saved to the database. Continuing with the `find_or_create_by` example, we
+now want the client named 'Nick':
 
 ```ruby
-nick = Client.where(:first_name => 'Nick').first_or_initialize(:locked => false)
-# => <Client id: nil, first_name: "Nick", orders_count: 0, locked: false, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
+nick = Client.find_or_initialize_by(first_name: 'Nick')
+# => <Client id: nil, first_name: "Nick", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">
 
 nick.persisted?
 # => false
@@ -1306,6 +1334,9 @@ nick.save
 # => true
 ```
 
+There is also a `first_or_initialize` method which does a similar thing
+for `first_or_create`.
+
 Finding by SQL
 --------------