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Semantic Search Made Easy With LangChain and MongoDB

Brian Leonard3 min read • Published Nov 30, 2023 • Updated Dec 11, 2023
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Enabling semantic search on user-specific data is a multi-step process that includes loading, transforming, embedding, and storing data before it can be queried.
LangChain Retrieval Flow
That graphic is from the team over at LangChain, whose goal is to provide a set of utilities to greatly simplify this process.
In this tutorial, we'll walk through each of these steps, using MongoDB Atlas as our Store. Specifically, we'll use the AT&T Wikipedia page as our data source. We'll then use libraries from LangChain to load, transform, embed, and store:
LangChain Storage Flow Example
Once the source is stored in MongoDB, we can retrieve the data that interests us:
LangChain Retrieval Flow


Quick start steps

  1. Get the code:
  1. Update with your MongoDB connection string and Open AI API key.
  2. Create a new Python environment
  1. Activate the new Python environment
  1. Install the requirements
  1. Load, transform, embed, and store
  1. Retrieve

The details

Load -> Transform -> Embed -> Store

Step 1: Load

There's no lack of sources of data — Slack, YouTube, Git, Excel, Reddit, Twitter, etc. — and LangChain provides a growing list of integrations that includes this list and many more.
For this exercise, we're going to use the WebBaseLoader to load the Wikipedia page for AT&T.

Step 2: Transform (Split)

Now that we have a bunch of text loaded, it needs to be split into smaller chunks so we can tease out the relevant portion based on our search query. For this example, we'll use the recommended RecursiveCharacterTextSplitter. As I have it configured, it attempts to split on paragraphs ("\n\n"), then sentences("(?<=\. )"), and then words (" ") using a chunk size of 1,000 characters. So if a paragraph doesn't fit into 1,000 characters, it will truncate at the next word it can fit to keep the chunk size under 1,000 characters. You can tune the chunk_size to your liking. Smaller numbers will lead to more documents, and vice-versa.

Step 3: Embed

Embedding is where you associate your text with an LLM to create a vector representation of that text. There are many options to choose from, such as OpenAI and Hugging Face, and LangChang provides a standard interface for interacting with all of them.
For this exercise, we're going to use the popular OpenAI embedding. Before proceeding, you'll need an API key for the OpenAI platform, which you will set in
We're simply going to load the embedder in this step. The real power comes when we store the embeddings in Step 4.

Step 4: Store

You'll need a vector database to store the embeddings, and lucky for you MongoDB fits that bill. Even luckier for you, the folks at LangChain have a MongoDB Atlas module that will do all the heavy lifting for you! Don't forget to add your MongoDB Atlas connection string to
You'll find the complete script in, which needs to be run once per data source (and you could easily modify the code to iterate over multiple data sources).

Step 5: Index the vector embeddings

The final step before we can query the data is to create a search index on the stored embeddings.
In the Atlas console and using the JSON editor, create a Search Index named vsearch_index with the following definition:
Create a Search Index - Configuration Method
Create a Search Index - JSON Editor


We can now run a search, using methods like similirity_search or max_marginal_relevance_search. That would return the relevant slice of data, which in our case would be an entire paragraph. However, we can continue to harness the power of the LLM to contextually compress the response so that it more directly tries to answer our question.


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Table of Contents
  • Prerequisites