Add docstring for graph vectorstores Link

libs/core/langchain_core/graph_vectorstores/links.py

@@ -8,10 +8,113 @@
 @beta()
 @dataclass(frozen=True)
 class Link:
-    """A link to/from a tag of a given tag.
+    """A link to/from a tag of a given kind.
 
-    Edges exist from nodes with an outgoing link to nodes with a matching incoming link.
-    """
+    Documents in a :class:`graph vector store <langchain_core.graph_vectorstores.base.GraphVectorStore>`
+    are connected via "links".
+    Links form a bipartite graph between documents and tags: documents are connected
+    to tags, and tags are connected to other documents.
+    When documents are retrieved from a graph vector store, a pair of documents are
+    connected with a depth of one if both documents are connected to the same tag.
+
+    Links have a ``kind`` property, used to namespace different tag identifiers.
+    For example a link to a keyword might use kind ``kw``, while a link to a URL might
+    use kind ``url``.
+    This allows the same tag value to be used in different contexts without causing
+    name collisions.
+
+    Links are directed. The directionality of links controls how the graph is
+    traversed at retrieval time.
+    For example, given documents ``A`` and ``B``, connected by links to tag ``T``:
+
+    +----------+----------+---------------------------------+
+    | A to T   | B to T   | Result                          |
+    +==========+==========+=================================+
+    | outgoing | incoming | Retrieval traverses from A to B |
+    +----------+----------+---------------------------------+
+    | incoming | incoming | No traversal from A to B        |
+    +----------+----------+---------------------------------+
+    | outgoing | incoming | No traversal from A to B        |
+    +----------+----------+---------------------------------+
+    | bidir    | incoming | Retrieval traverses from A to B |
+    +----------+----------+---------------------------------+
+    | bidir    | outgoing | No traversal from A to B        |
+    +----------+----------+---------------------------------+
+    | outgoing | bidir    | Retrieval traverses from A to B |
+    +----------+----------+---------------------------------+
+    | incoming | bidir    | No traversal from A to B        |
+    +----------+----------+---------------------------------+
+
+    Directed links make it possible to describe relationships such as term
+    references / definitions: term definitions are generally relevant to any documents
+    that use the term, but the full set of documents using a term generally aren't
+    relevant to the term's definition.
+
+    .. seealso::
+
+        - :mod:`How to use a graph vector store <langchain_community.graph_vectorstores>`
+        - :class:`How to link Documents on hyperlinks in HTML <langchain_community.graph_vectorstores.extractors.html_link_extractor.HtmlLinkExtractor>`
+        - :class:`How to link Documents on common keywords (using KeyBERT) <langchain_community.graph_vectorstores.extractors.keybert_link_extractor.KeybertLinkExtractor>`
+        - :class:`How to link Documents on common named entities (using GliNER) <langchain_community.graph_vectorstores.extractors.gliner_link_extractor.GLiNERLinkExtractor>`
+
+    How to add links to a Document
+    ==============================
+
+    How to create links
+    -------------------
+
+    You can create links using the Link class's constructors :meth:`incoming`,
+    :meth:`outgoing`, and :meth:`bidir`::
+
+        from langchain_core.graph_vectorstores.links import Link
+
+        print(Link.bidir(kind="location", tag="Paris"))
+
+    .. code-block:: output
+
+        Link(kind='location', direction='bidir', tag='Paris')
+
+    Extending documents with links
+    ------------------------------
+
+    Now that we know how to create links, let's associate them with some documents.
+    These edges will strengthen the connection between documents that share a keyword
+    when using a graph vector store to retrieve documents.
+
+    First, we'll load some text and chunk it into smaller pieces.
+    Then we'll add a link to each document to link them all together::
+
+        from langchain_community.document_loaders import TextLoader
+        from langchain_core.graph_vectorstores.links import add_links
+        from langchain_text_splitters import CharacterTextSplitter
+
+        loader = TextLoader("state_of_the_union.txt")
+
+        raw_documents = loader.load()
+        text_splitter = CharacterTextSplitter(chunk_size=1000, chunk_overlap=0)
+        documents = text_splitter.split_documents(raw_documents)
+
+        for doc in documents:
+            add_links(doc, Link.bidir(kind="genre", tag="oratory"))
+
+        print(documents[0].metadata)
+
+    .. code-block:: output
+
+        {'source': 'state_of_the_union.txt', 'links': [Link(kind='genre', direction='bidir', tag='oratory')]}
+
+    As we can see, each document's metadata now includes a bidirectional link to the
+    genre ``oratory``.
+
+    The documents can then be added to a graph vector store::
+
+        from langchain_community.graph_vectorstores import CassandraGraphVectorStore
+
+        graph_vectorstore = CassandraGraphVectorStore.from_documents(
+            documents=documents, embeddings=...
+        )
+
+    """  # noqa: E501
 
     kind: str
     """The kind of link. Allows different extractors to use the same tag name without
@@ -23,17 +126,32 @@ class Link:
 
     @staticmethod
     def incoming(kind: str, tag: str) -> "Link":
-        """Create an incoming link."""
+        """Create an incoming link.
+
+        Args:
+            kind: the link kind.
+            tag: the link tag.
+        """
         return Link(kind=kind, direction="in", tag=tag)
 
     @staticmethod
     def outgoing(kind: str, tag: str) -> "Link":
-        """Create an outgoing link."""
+        """Create an outgoing link.
+
+        Args:
+            kind: the link kind.
+            tag: the link tag.
+        """
         return Link(kind=kind, direction="out", tag=tag)
 
     @staticmethod
     def bidir(kind: str, tag: str) -> "Link":
-        """Create a bidirectional link."""
+        """Create a bidirectional link.
+
+        Args:
+            kind: the link kind.
+            tag: the link tag.
+        """
         return Link(kind=kind, direction="bidir", tag=tag)