Add CHANGELOG for #38029

activerecord/CHANGELOG.md

+*   Don't allow mutations on the datbase configurations hash.
+
+    Freeze the configurations hash to disallow directly changing the configurations hash. If applications need to change the hash, for example to create adatabases for parallelization, they should use the `DatabaseConfig` object directly.
+
+    Before:
+
+    ```ruby
+    @db_config = ActiveRecord::Base.configurations.configs_for(env_name: "test", spec_name: "primary")
+    @db_config.configuration_hash.merge!(idle_timeout: "0.02")
+    ```
+
+    After:
+
+    ```ruby
+    @db_config = ActiveRecord::Base.configurations.configs_for(env_name: "test", spec_name: "primary")
+    config = @db_config.configuration_hash.merge(idle_timeout: "0.02")
+    db_config = ActiveRecord::DatabaseConfigurations::HashConfig.new(@db_config.env_name, @db_config.spec_name, config)
+    ```
+
+    *Eileen M. Uchitelle*, *John Crepezzi*
+
 *   Remove `:connection_id` from the `sql.active_record` notification.
 
     *Aaron Patterson*, *Rafael Mendonça França*