Adds helper for `find_or_create_by` in transaction

This allows us to call `find_or_create_by` on all models and scopes.

app/models/application_record.rb

@@ -6,4 +6,12 @@ class ApplicationRecord < ActiveRecord::Base
   def self.id_in(ids)
     where(id: ids)
   end
+
+  def self.safe_find_or_create_by(*args)
+    transaction(requires_new: true) do
+      find_or_create_by(*args)
+    end
+  rescue ActiveRecord::RecordNotUnique
+    retry
+  end
 end

doc/development/sql.md

@@ -256,32 +256,12 @@ violation, for example.
 
 Using transactions does not solve this problem.
 
-The following pattern should be used to avoid the problem:
+To solve this we've added the `ApplicationRecord.safe_find_or_create_by`.
 
-```ruby
-Project.transaction do
-  begin
-    User.find_or_create_by(username: "foo")
-  rescue ActiveRecord::RecordNotUnique
-    retry
-  end
-end
-```
-
-If the above block is run inside a transaction and hits the race
-condition, the transaction is aborted and we cannot simply retry (any
-further queries inside the aborted transaction are going to fail). We
-can employ [nested transactions](http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html#module-ActiveRecord::Transactions::ClassMethods-label-Nested+transactions)
-here to only rollback the "inner transaction". Note that `requires_new: true` is required here.
+This method can be used just as you would the normal
+`find_or_create_by` but it wraps the call in a *new* transaction and
+retries if it were to fail because of an
+`ActiveRecord::RecordNotUnique` error.
 
-```ruby
-Project.transaction do
-  begin
-    User.transaction(requires_new: true) do
-      User.find_or_create_by(username: "foo")
-    end
-  rescue ActiveRecord::RecordNotUnique
-    retry
-  end
-end
-```
+To be able to use this method, make sure the model you want to use
+this on inherits from `ApplicationRecord`.

spec/models/application_record_spec.rb

@@ -10,4 +10,14 @@ describe ApplicationRecord do
       expect(User.id_in(records.last(2).map(&:id))).to eq(records.last(2))
     end
   end
+
+  describe '#safe_find_or_create_by' do
+    it 'creates the user avoiding race conditions' do
+      expect(Suggestion).to receive(:find_or_create_by).and_raise(ActiveRecord::RecordNotUnique)
+      allow(Suggestion).to receive(:find_or_create_by).and_call_original
+
+      expect { Suggestion.safe_find_or_create_by(build(:suggestion).attributes) }
+        .to change { Suggestion.count }.by(1)
+    end
+  end
 end