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HomeBlogMachine Learning

Build a RAG Application with LangChain and Python: Complete Tutorial

Mohammed Aman
Mohammed Aman
date 23 June 2025
time 13 min read

Build a RAG Application with LangChain and Python: Complete Tutorial

Retrieval-Augmented Generation (RAG) lets you build AI that answers questions from your private documents, databases, and knowledge bases without expensive fine-tuning. This tutorial builds a production-ready RAG app from scratch.

Build a RAG Application with LangChain and Python: Complete Tutorial

What is RAG and Why Does It Matter?

Retrieval-Augmented Generation solves the fundamental problem of LLMs: they only know what they were trained on. RAG gives an LLM access to your specific knowledge — company documentation, product manuals, research papers, internal databases — at query time, without retraining the model.

The alternative, fine-tuning, requires thousands of labeled examples, GPU time, and re-training whenever your knowledge changes. RAG requires none of that. When a user asks a question, you retrieve the relevant documents from your knowledge base, inject them into the prompt, and let the LLM generate an answer grounded in your actual data. The result is dramatically more accurate and up-to-date than a standalone LLM.

How RAG Works Under the Hood

The RAG pipeline has two phases. Indexing: load your documents, split them into chunks of 500 to 1000 tokens, convert each chunk into a vector embedding (a high-dimensional numerical representation of meaning) using an embedding model, and store those vectors in a vector database. This happens once, when you ingest new content.

Retrieval and generation: when a user submits a query, embed the query using the same embedding model, perform a similarity search in the vector database to find the most relevant chunks, inject those chunks into the LLM prompt as context, and have the LLM generate an answer based on the retrieved information. The LLM cites the source documents, making answers verifiable.

Building the Indexing Pipeline with LangChain

LangChain's document loaders handle PDFs, web pages, Notion, Google Drive, SQL databases, and dozens of other sources. The RecursiveCharacterTextSplitter splits documents into overlapping chunks, preserving context at boundaries. OpenAIEmbeddings or HuggingFaceEmbeddings convert text to vectors. Chroma, FAISS, or Pinecone stores and queries those vectors.

For production deployments, Supabase with the pgvector extension is an excellent choice — it gives you a managed PostgreSQL database with vector similarity search, eliminating the need for a separate vector database. LlamaIndex provides a simpler alternative to LangChain specifically optimized for RAG use cases, with less boilerplate.

Production Considerations and Advanced Techniques

Chunking strategy significantly impacts RAG quality. Chunks that are too small lose context. Chunks that are too large dilute relevance. 512 tokens with 50-token overlap is a good starting point. Experiment with semantic chunking — splitting at topic boundaries rather than fixed character counts — for more coherent retrieval.

Hybrid search combines vector similarity with keyword search (BM25) to handle both semantic queries (what are the benefits of exercise) and specific fact queries (what is the dosage of medication X). Re-ranking with a cross-encoder model improves precision by re-scoring the top-k retrieved documents. Evaluate RAG quality with the RAGAS framework, which measures answer faithfulness, context precision, and context recall automatically.

  • Tags:
  • RAG
  • LangChain
  • Python
  • LLM
  • Vector Database
  • AI Development
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Mohammed Aman

Mohammed Aman

Tech blogger covering AI, coding, and the future of software. Founder of CodeWithBeast.

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