How to map nodes and relationships

Through this guide, you will learn how to map Python classes to nodes and relationships in a graph database.

Make sure you have a running Memgraph instance. If you're not sure how to run Memgraph, check out the Memgraph Quick start.

First, do all necessary imports and create an instance of the database:

from gqlalchemy import Memgraph, Node, Relationship, Field
from typing import Optional

After that, you instantiate Memgraph and create classes representing the nodes and relationships. You are creating User and Streamer nodes, where every Streamer is also a User, but not every User is a Streamer.

db = Memgraph()

class User(Node, index=True, db=db):
    id: str = Field(index=True, exist=True, unique=True, db=db)

class Streamer(User):
    id: str = Field(index=True, exist=True, unique=True, db=db)
    username: Optional[str] = Field(index=True, exist=True, unique=True, db=db)
    url: Optional[str] = Field()
    followers: Optional[int] = Field()
    createdAt: Optional[str] = Field()
    totalViewCount: Optional[int] = Field()
    description: Optional[str] = Field()

important

You can also use this feature with Neo4j:

db = Neo4j(host="localhost", port="7687", username="neo4j", password="test")

Node is a Python class which maps to a graph object in Memgraph. User and Streamer are classes which inherit from Node and they map to a label in the graph database. Class User maps to a single :User label, while class Streamer maps to multiple labels :Streamer:User, since it inherits from the User class. Notice that the index argument in class definition is set to True. That means that you have created a label index on the User label. If you create a node with the label User, that node has a property id, which is also indexed (label-property index) and a unique string. With the help of Field(), you can define those constraints on the properties and define to which database that property will be saved. Argument index=True is added to the property id, which is both in the User and Streamer class. Because Streamer inherits from User, the property id will be indexed on nodes labeled with User. In the Cypher query language, that means the query CREATE INDEX ON :User(id); will be executed.

In a similar way, you can create a relationship:

class ChatsWith(Relationship, type="CHATS_WITH"):
    lastChatted: Optional[str] = Field()

Now you have created a relationship of type CHATS_WITH. This relationship has the property lastChatted, which is optional. If you want to create a relationship without any properties, you can do that with:

class ChatsWith(Relationship, type="CHATS_WITH"):
    pass

Hopefully this guide has taught you how to map nodes and relationships. If you have any more questions, join our community and ping us on Discord.