The problem isn't the detection but the deadlock itself

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -753,7 +753,7 @@ module ActiveRecord
         when ER_DATA_TOO_LONG
           ValueTooLong.new(message)
         when ER_LOCK_DEADLOCK
-          DeadlockDetected.new(message)
+          Deadlocked.new(message)
         else
           super
         end

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -407,7 +407,7 @@ module ActiveRecord
         FOREIGN_KEY_VIOLATION = "23503"
         UNIQUE_VIOLATION      = "23505"
         SERIALIZATION_FAILURE = "40001"
-        DEADLOCK_DETECTED     = "40P01"
+        DEADLOCKED            = "40P01"
 
         def translate_exception(exception, message)
           return exception unless exception.respond_to?(:result)
@@ -421,8 +421,8 @@ module ActiveRecord
             ValueTooLong.new(message)
           when SERIALIZATION_FAILURE
             SerializationFailure.new(message)
-          when DEADLOCK_DETECTED
-            DeadlockDetected.new(message)
+          when DEADLOCKED
+            Deadlocked.new(message)
           else
             super
           end

activerecord/lib/active_record/errors.rb

@@ -300,9 +300,9 @@ module ActiveRecord
   class SerializationFailure < TransactionRollbackError
   end
 
-  # DeadlockDetected will be raised when a transaction is rolled
+  # Deadlocked will be raised when a transaction is rolled
   # back by the database when a deadlock is encountered.
-  class DeadlockDetected < TransactionRollbackError
+  class Deadlocked < TransactionRollbackError
   end
 
   # IrreversibleOrderError is raised when a relation's order is too complex for

activerecord/test/cases/adapters/mysql2/transaction_test.rb

@@ -27,8 +27,8 @@ module ActiveRecord
       @connection.drop_table 'samples', if_exists: true
     end
 
-    test "raises DeadlockDetected when a deadlock is encountered" do
-      assert_raises(ActiveRecord::DeadlockDetected) do
+    test "raises Deadlocked when a deadlock is encountered" do
+      assert_raises(ActiveRecord::Deadlocked) do
         s1 = Sample.create value: 1
         s2 = Sample.create value: 2
 

activerecord/test/cases/adapters/postgresql/transaction_test.rb

@@ -58,9 +58,9 @@ module ActiveRecord
       end
     end
 
-    test "raises DeadlockDetected when a deadlock is encountered" do
+    test "raises Deadlocked when a deadlock is encountered" do
       with_warning_suppression do
-        assert_raises(ActiveRecord::DeadlockDetected) do
+        assert_raises(ActiveRecord::Deadlocked) do
           s1 = Sample.create value: 1
           s2 = Sample.create value: 2