Make document serializable, create utility to create a docstore (#9674)

This PR makes the following changes:

1. Documents become serializable using langhchain serialization
2. Make a utility to create a docstore kw store

Will help to address issue here:
https://github.com/langchain-ai/langchain/issues/9345

libs/langchain/langchain/schema/document.py

@@ -17,6 +17,11 @@ class Document(Serializable):
         documents, etc.).
     """
 
+    @property
+    def lc_serializable(self) -> bool:
+        """Return whether or not the class is serializable."""
+        return True
+
 
 class BaseDocumentTransformer(ABC):
     """Abstract base class for document transformation systems.

libs/langchain/langchain/storage/__init__.py

@@ -6,6 +6,7 @@ to a simple key-value interface.
 The primary goal of these storages is to support implementation of caching.
 """
 
+from langchain.storage._lc_store import create_kv_docstore, create_lc_store
 from langchain.storage.encoder_backed import EncoderBackedStore
 from langchain.storage.file_system import LocalFileStore
 from langchain.storage.in_memory import InMemoryStore
@@ -16,4 +17,6 @@ __all__ = [
     "InMemoryStore",
     "LocalFileStore",
     "RedisStore",
+    "create_lc_store",
+    "create_kv_docstore",
 ]

libs/langchain/langchain/storage/_lc_store.py

@@ -0,0 +1,88 @@
+"""Create a key-value store for any langchain serializable object."""
+from typing import Callable, Optional
+
+from langchain.load.dump import dumps
+from langchain.load.load import loads
+from langchain.load.serializable import Serializable
+from langchain.schema import BaseStore, Document
+from langchain.storage.encoder_backed import EncoderBackedStore
+
+
+def _dump_as_bytes(obj: Serializable) -> bytes:
+    """Return a bytes representation of a document."""
+    return dumps(obj).encode("utf-8")
+
+
+def _dump_document_as_bytes(obj: Document) -> bytes:
+    """Return a bytes representation of a document."""
+    if not isinstance(obj, Document):
+        raise TypeError("Expected a Document instance")
+    return dumps(obj).encode("utf-8")
+
+
+def _load_document_from_bytes(serialized: bytes) -> Document:
+    """Return a document from a bytes representation."""
+    obj = loads(serialized.decode("utf-8"))
+    if not isinstance(obj, Document):
+        raise TypeError(f"Expected a Document instance. Got {type(obj)}")
+    return obj
+
+
+def _load_from_bytes(serialized: bytes) -> Serializable:
+    """Return a document from a bytes representation."""
+    return loads(serialized.decode("utf-8"))
+
+
+def _identity(x: str) -> str:
+    """Return the same object."""
+    return x
+
+
+# PUBLIC API
+
+
+def create_lc_store(
+    store: BaseStore[str, bytes],
+    *,
+    key_encoder: Optional[Callable[[str], str]] = None,
+) -> BaseStore[str, Serializable]:
+    """Create a store for langchain serializable objects from a bytes store.
+
+    Args:
+        store: A bytes store to use as the underlying store.
+        key_encoder: A function to encode keys; if None uses identity function.
+
+    Returns:
+        A key-value store for documents.
+    """
+    return EncoderBackedStore(
+        store,
+        key_encoder or _identity,
+        _dump_as_bytes,
+        _load_from_bytes,
+    )
+
+
+def create_kv_docstore(
+    store: BaseStore[str, bytes],
+    *,
+    key_encoder: Optional[Callable[[str], str]] = None,
+) -> BaseStore[str, Document]:
+    """Create a store for langchain Document objects from a bytes store.
+
+    This store does run time type checking to ensure that the values are
+    Document objects.
+
+    Args:
+        store: A bytes store to use as the underlying store.
+        key_encoder: A function to encode keys; if None uses identity function.
+
+    Returns:
+        A key-value store for documents.
+    """
+    return EncoderBackedStore(
+        store,
+        key_encoder or _identity,
+        _dump_document_as_bytes,
+        _load_document_from_bytes,
+    )

libs/langchain/tests/unit_tests/storage/test_lc_store.py

@@ -0,0 +1,36 @@
+import tempfile
+from typing import Generator, cast
+
+import pytest
+
+from langchain.schema import Document
+from langchain.storage._lc_store import create_kv_docstore, create_lc_store
+from langchain.storage.file_system import LocalFileStore
+
+
+@pytest.fixture
+def file_store() -> Generator[LocalFileStore, None, None]:
+    # Create a temporary directory for testing
+    with tempfile.TemporaryDirectory() as temp_dir:
+        # Instantiate the LocalFileStore with the temporary directory as the root path
+        store = LocalFileStore(temp_dir)
+        yield store
+
+
+def test_create_lc_store(file_store: LocalFileStore) -> None:
+    """Test that a docstore is created from a base store."""
+    docstore = create_lc_store(file_store)
+    docstore.mset([("key1", Document(page_content="hello", metadata={"key": "value"}))])
+    fetched_doc = cast(Document, docstore.mget(["key1"])[0])
+    assert fetched_doc.page_content == "hello"
+    assert fetched_doc.metadata == {"key": "value"}
+
+
+def test_create_kv_store(file_store: LocalFileStore) -> None:
+    """Test that a docstore is created from a base store."""
+    docstore = create_kv_docstore(file_store)
+    docstore.mset([("key1", Document(page_content="hello", metadata={"key": "value"}))])
+    fetched_doc = docstore.mget(["key1"])[0]
+    assert isinstance(fetched_doc, Document)
+    assert fetched_doc.page_content == "hello"
+    assert fetched_doc.metadata == {"key": "value"}