RAG with Knowledge Graphs

1. Knowledge Graph Fundamentals

A knowledge graph (KG) is a structured representation of real-world entities and the relationships between them. Unlike documents stored as unstructured text, knowledge graphs encode information as a network of nodes (entities) connected by typed edges (relations). This structure enables precise queries about relationships, paths, and patterns that would require complex reasoning over unstructured text.

1.1 Entities, Relations, and Triples

The fundamental unit of a knowledge graph is the triple, expressed as (subject, predicate, object) or equivalently (head, relation, tail). For example, (Albert Einstein, bornIn, Ulm) encodes a single fact. A knowledge graph is simply a collection of such triples, forming a directed labeled graph where entities are nodes and relations are edge labels.

1.2 RDF vs. Property Graphs

Two main formalisms exist for representing knowledge graphs. RDF (Resource Description Framework) is a W3C standard that represents all knowledge as triples and uses URIs to identify entities and relations. RDF is queried using SPARQL and is the foundation of the Semantic Web (Wikidata, DBpedia). Property graphs allow nodes and edges to carry arbitrary key-value properties, offering greater flexibility. Property graphs are queried using Cypher (Neo4j) or Gremlin and are more commonly used in industry applications.

2. Building Knowledge Graphs with LLMs

Traditionally, knowledge graph construction required extensive manual curation or specialized NLP pipelines for entity extraction and relation classification. LLMs have dramatically simplified this process by extracting structured triples directly from unstructured text through carefully designed prompts.

from openai import OpenAI
import json

client = OpenAI()

def extract_triples(text, entity_types=None):
    """Extract knowledge graph triples from text using an LLM."""
    type_hint = ""
    if entity_types:
        type_hint = f"\nFocus on these entity types: {', '.join(entity_types)}"

    response = client.chat.completions.create(
        model="gpt-4o",
        messages=[{
            "role": "system",
            "content": f"""Extract knowledge graph triples from the text.
Return a JSON array of objects with keys: head, relation, tail.
Use concise, canonical entity names.
Use lowercase relation names with underscores.{type_hint}

Example output:
[
  {{"head": "OpenAI", "relation": "developed", "tail": "GPT-4"}},
  {{"head": "GPT-4", "relation": "is_a", "tail": "language model"}}
]"""
        }, {
            "role": "user",
            "content": text
        }],
        response_format={"type": "json_object"},
        temperature=0.0
    )

    result = json.loads(response.choices[0].message.content)
    return result.get("triples", result)

2.1 Storing Triples in Neo4j

from neo4j import GraphDatabase

class KnowledgeGraphStore:
    """Store and query knowledge graph triples in Neo4j."""

    def __init__(self, uri, user, password):
        self.driver = GraphDatabase.driver(uri, auth=(user, password))

    def add_triples(self, triples):
        """Insert triples as nodes and relationships."""
        with self.driver.session() as session:
            for triple in triples:
                session.run("""
                    MERGE (h:Entity {name: $head})
                    MERGE (t:Entity {name: $tail})
                    MERGE (h)-[r:RELATES {type: $relation}]->(t)
                """, head=triple["head"],
                     tail=triple["tail"],
                     relation=triple["relation"])

    def find_neighbors(self, entity, max_hops=2):
        """Find all entities within N hops of the given entity."""
        with self.driver.session() as session:
            result = session.run(f"""
                MATCH path = (e:Entity {{name: $entity}})
                      -[*1..{max_hops}]-(neighbor)
                RETURN path
                LIMIT 50
            """, entity=entity)
            return [record["path"] for record in result]

    def query_relationship(self, head, tail):
        """Find relationship paths between two entities."""
        with self.driver.session() as session:
            result = session.run("""
                MATCH path = shortestPath(
                    (h:Entity {name: $head})-[*]-(t:Entity {name: $tail})
                )
                RETURN path
            """, head=head, tail=tail)
            return [record["path"] for record in result]

3. Graph Embeddings

Graph embeddings map entities and relations from a knowledge graph into continuous vector spaces, enabling similarity-based retrieval, link prediction (predicting missing triples), and integration with dense retrieval systems. The key challenge is preserving the structural properties of the graph in the embedding space.

3.1 TransE

TransE (Bordes et al., 2013) is the foundational graph embedding model. It represents relations as translations in embedding space: if (h, r, t) is a true triple, then the embedding of the head entity plus the embedding of the relation should approximately equal the embedding of the tail entity: h + r ≈ t. The model is trained by minimizing a margin-based loss that pushes correct triples closer than corrupted triples.

3.2 DistMult

DistMult (Yang et al., 2015) models relations as diagonal matrices (represented as vectors) and scores triples using a bilinear product: score(h, r, t) = h · diag(r) · t. This is equivalent to an element-wise product followed by a sum. DistMult is simple and effective but cannot model asymmetric relations (it gives the same score to (h, r, t) and (t, r, h)), which limits its applicability.

import torch
import torch.nn as nn

class TransE(nn.Module):
    """TransE knowledge graph embedding model."""

    def __init__(self, num_entities, num_relations, dim=128):
        super().__init__()
        self.entity_emb = nn.Embedding(num_entities, dim)
        self.relation_emb = nn.Embedding(num_relations, dim)

        # Initialize with uniform distribution
        nn.init.uniform_(self.entity_emb.weight, -6/dim**0.5, 6/dim**0.5)
        nn.init.uniform_(self.relation_emb.weight, -6/dim**0.5, 6/dim**0.5)

    def forward(self, heads, relations, tails):
        """Score triples: lower distance = more plausible."""
        h = self.entity_emb(heads)
        r = self.relation_emb(relations)
        t = self.entity_emb(tails)

        # TransE scoring: ||h + r - t||
        score = torch.norm(h + r - t, p=2, dim=-1)
        return score

    def margin_loss(self, pos_score, neg_score, margin=1.0):
        """Margin-based ranking loss."""
        return torch.relu(margin + pos_score - neg_score).mean()

4. GraphRAG

GraphRAG (Microsoft Research, 2024) represents a paradigm shift in how knowledge graphs are used for RAG. Instead of querying a pre-existing knowledge graph, GraphRAG builds a knowledge graph from the document corpus itself, then uses community detection to create hierarchical summaries of entity clusters. This enables both local retrieval (finding specific facts) and global retrieval (answering questions that require synthesizing information across the entire corpus).

4.1 The GraphRAG Pipeline

1. Entity and relation extraction: An LLM extracts entities and relationships from each document chunk, building a corpus-wide knowledge graph.
2. Community detection: The Leiden algorithm partitions the graph into communities of closely connected entities, creating a hierarchy from fine-grained clusters to broad themes.
3. Community summarization: An LLM generates a summary for each community, capturing the key entities, relationships, and themes within that cluster.
4. Query routing: At query time, the system determines whether to use local search (entity-based retrieval for specific questions) or global search (community summary-based retrieval for broad questions).

5. Hybrid KG + Vector Retrieval

The most effective production systems combine knowledge graph traversal with vector search. The knowledge graph provides structured multi-hop reasoning and entity-aware retrieval, while vector search provides semantic similarity for unstructured content that the KG does not cover.

def hybrid_kg_vector_retrieve(query, kg_store, vector_collection, k=5):
    """Combine KG traversal with vector search."""

    # Step 1: Extract entities from query
    entities = extract_entities_from_query(query)

    # Step 2: KG retrieval (structured paths)
    kg_context = []
    for entity in entities:
        neighbors = kg_store.find_neighbors(entity, max_hops=2)
        kg_context.extend(format_paths(neighbors))

    # Step 3: Vector retrieval (semantic similarity)
    vector_results = vector_collection.query(
        query_texts=[query],
        n_results=k
    )
    vector_context = vector_results["documents"][0]

    # Step 4: Combine both context types
    combined_context = f"""Structured Knowledge (from Knowledge Graph):
{chr(10).join(kg_context)}

Relevant Documents (from semantic search):
{chr(10).join(vector_context)}"""

    return combined_context

6. When to Use Knowledge Graph RAG