Merge configurable fields with previous checkpoint config before each run (#510)

* Merge configurable fields with previous checkpoint config.

* Update update_state() and aupdate_state() to merge configurable fields with previous checkpoint config.

* Update tests to verify that all checkpoint metadata contain the expected configurable field keys. This assertion is needed because a run can have an arbitrary number of steps based on the construction of the graph.

tests/memory_assert.py

@@ -1,6 +1,9 @@
+import asyncio
 from collections import defaultdict
 from typing import Any, Optional
 
+from langchain_core.runnables import RunnableConfig
+
 from langgraph.checkpoint.base import (
     Checkpoint,
     CheckpointMetadata,
@@ -46,3 +49,63 @@ class MemorySaverAssertImmutable(MemorySaver):
         )
         # call super to write checkpoint
         return super().put(config, checkpoint, metadata)
+
+
+class MemorySaverAssertCheckpointMetadata(MemorySaver):
+    """This custom checkpointer is for verifying that a run's configurable
+    fields are merged with the previous checkpoint config for each step in
+    the run. This is the desired behavior. Because the checkpointer's (a)put()
+    method is called for each step, the implementation of this checkpointer
+    should produce a side effect that can be asserted.
+    """
+
+    serde = NoopSerializer()
+
+    def __init__(
+        self,
+        *,
+        serde: Optional[SerializerProtocol] = None,
+    ) -> None:
+        super().__init__(serde=serde)
+
+    def put(
+        self,
+        config: RunnableConfig,
+        checkpoint: Checkpoint,
+        metadata: Optional[CheckpointMetadata] = None,
+    ) -> None:
+        """The implementation of put() merges config["configurable"] (a run's
+        configurable fields) with the metadata field. The state of the
+        checkpoint metadata can be asserted to confirm that the run's
+        configurable fields were merged with the previous checkpoint config.
+        """
+        configurable = config["configurable"].copy()
+
+        # remove thread_ts to make testing simpler
+        configurable.pop("thread_ts", None)
+
+        self.storage[config["configurable"]["thread_id"]].update(
+            {
+                checkpoint["id"]: (
+                    self.serde.dumps(checkpoint),
+                    # merge configurable fields and metadata
+                    self.serde.dumps({**configurable, **metadata}),
+                )
+            }
+        )
+        return {
+            "configurable": {
+                "thread_id": config["configurable"]["thread_id"],
+                "thread_ts": checkpoint["id"],
+            }
+        }
+
+    async def aput(
+        self,
+        config: RunnableConfig,
+        checkpoint: Checkpoint,
+        metadata: CheckpointMetadata,
+    ) -> RunnableConfig:
+        return await asyncio.get_running_loop().run_in_executor(
+            None, self.put, config, checkpoint, metadata
+        )