Merge pull request rails#55407 from kirs/current-transaction-isolation

Provide a method to check current transaction's isolation level

Author: byroot

activerecord/CHANGELOG.md

@@ -1,3 +1,33 @@
+*   Add `connection.current_transaction.isolation` API to check current transaction's isolation level.
+
+    Returns the isolation level if it was explicitly set via the `isolation:` parameter
+    or through `ActiveRecord.with_transaction_isolation_level`, otherwise returns `nil`.
+    Nested transactions return the parent transaction's isolation level.
+
+    ```ruby
+    # Returns nil when no transaction
+    User.connection.current_transaction.isolation # => nil
+
+    # Returns explicitly set isolation level
+    User.transaction(isolation: :serializable) do
+      User.connection.current_transaction.isolation # => :serializable
+    end
+
+    # Returns nil when isolation not explicitly set
+    User.transaction do
+      User.connection.current_transaction.isolation # => nil
+    end
+
+    # Nested transactions inherit parent's isolation
+    User.transaction(isolation: :read_committed) do
+      User.transaction do
+        User.connection.current_transaction.isolation # => :read_committed
+      end
+    end
+    ```
+
+    *Kir Shatrov*
+
 *   Emit a warning for pg gem < 1.6.0 when using PostgreSQL 18+
 
     *Yasuo Honda*

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -112,6 +112,7 @@ def state; end
       def closed?; true; end
       def open?; false; end
       def joinable?; false; end
+      def isolation; nil; end
       def add_record(record, _ = true); end
       def restartable?; false; end
       def dirty?; false; end
@@ -150,6 +151,11 @@ def after_rollback
 
       delegate :invalidate!, :invalidated?, to: :@state
 
+      # Returns the isolation level if it was explicitly set, nil otherwise
+      def isolation
+        @isolation_level
+      end
+
       def initialize(connection, isolation: nil, joinable: true, run_commit_callbacks: false)
         super()
         @connection = connection
@@ -386,7 +392,7 @@ def initialize(connection, parent_transaction, **options)
         @parent.state.add_child(@state)
       end
 
-      delegate :materialize!, :materialized?, :restart, to: :@parent
+      delegate :materialize!, :materialized?, :restart, :isolation, to: :@parent
 
       def rollback
         @state.rollback!
@@ -405,6 +411,7 @@ class SavepointTransaction < Transaction
       def initialize(connection, savepoint_name, parent_transaction, **options)
         super(connection, **options)
 
+        @parent_transaction = parent_transaction
         parent_transaction.state.add_child(@state)
 
         if isolation_level
@@ -414,6 +421,11 @@ def initialize(connection, savepoint_name, parent_transaction, **options)
         @savepoint_name = savepoint_name
       end
 
+      # Delegates to parent transaction's isolation level
+      def isolation
+        @parent_transaction.isolation
+      end
+
       def materialize!
         connection.create_savepoint(savepoint_name)
         super

activerecord/test/cases/transaction_isolation_test.rb

@@ -14,6 +14,10 @@ class Tag < ActiveRecord::Base
         Tag.transaction(isolation: :serializable) { Tag.lease_connection.materialize_transactions }
       end
     end
+
+    test "current_transaction.isolation returns nil when no transaction" do
+      assert_nil Tag.lease_connection.current_transaction.isolation
+    end
   end
 end
 
@@ -252,6 +256,30 @@ class Dog < ARUnit2Model
       end
     end
 
+    test "current_transaction.isolation returns nil when no transaction" do
+      assert_nil Tag.lease_connection.current_transaction.isolation
+    end
+
+    test "current_transaction.isolation returns explicitly set isolation level" do
+      Tag.transaction(isolation: :read_committed) do
+        assert_equal :read_committed, Tag.lease_connection.current_transaction.isolation
+      end
+    end
+
+    test "current_transaction.isolation returns parent isolation for nested transactions" do
+      Tag.transaction(isolation: :read_committed) do
+        Tag.transaction do
+          assert_equal :read_committed, Tag.lease_connection.current_transaction.isolation
+        end
+      end
+    end
+
+    test "current_transaction.isolation returns nil for transactions without explicit isolation" do
+      Tag.transaction do
+        assert_nil Tag.lease_connection.current_transaction.isolation
+      end
+    end
+
     private
       def assert_begin_isolation_level_event(events, isolation: "READ COMMITTED", count: 1)
         if current_adapter?(:PostgreSQLAdapter)