wip: add method for both docstore and embeddings (#119)

this will break atm but wanted to get thoughts on implementation.

1. should add() be on docstore interface?
2. should InMemoryDocstore change to take a list of documents as init?
(makes this slightly easier to implement in FAISS -- if we think it is
less clean then could expose a method to get the number of documents
currently in the dict, and perform the logic of creating the necessary
dictionary in the FAISS.add_texts method.

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

tests/unit_tests/docstore/test_inmemory.py

@@ -1,4 +1,5 @@
 """Test in memory docstore."""
+import pytest
 
 from langchain.docstore.document import Document
 from langchain.docstore.in_memory import InMemoryDocstore
@@ -19,3 +20,37 @@ def test_document_not_found() -> None:
     docstore = InMemoryDocstore(_dict)
     output = docstore.search("bar")
     assert output == "ID bar not found."
+
+
+def test_adding_document() -> None:
+    """Test that documents are added correctly."""
+    _dict = {"foo": Document(page_content="bar")}
+    docstore = InMemoryDocstore(_dict)
+    new_dict = {"bar": Document(page_content="foo")}
+    docstore.add(new_dict)
+
+    # Test that you can find new document.
+    foo_output = docstore.search("bar")
+    assert isinstance(foo_output, Document)
+    assert foo_output.page_content == "foo"
+
+    # Test that old document is the same.
+    bar_output = docstore.search("foo")
+    assert isinstance(bar_output, Document)
+    assert bar_output.page_content == "bar"
+
+
+def test_adding_document_already_exists() -> None:
+    """Test that error is raised if document id already exists."""
+    _dict = {"foo": Document(page_content="bar")}
+    docstore = InMemoryDocstore(_dict)
+    new_dict = {"foo": Document(page_content="foo")}
+
+    # Test that error is raised.
+    with pytest.raises(ValueError):
+        docstore.add(new_dict)
+
+    # Test that old document is the same.
+    bar_output = docstore.search("foo")
+    assert isinstance(bar_output, Document)
+    assert bar_output.page_content == "bar"