How to load nodes and relationships

Through this guide, you will learn how to load nodes and relationships from a graph database. First, perform all necessary imports and create an instance of the database:

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

db = Memgraph()

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

important

You can also use this feature with Neo4j:

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

Let's say you have defined Streamer, Team and IsPartOf classes:

class Streamer(Node):
    id: str = Field(index=True, unique=True, db=db)
    name: Optional[str] = Field(index=True, exists=True, unique=True, db=db)

class Team(Node):
    name: str = Field(unique=True, db=db)

class IsPartOf(Relationship, type="IS_PART_OF"):
    date: Optional[str] = Field()

There are two nodes already saved in the database:

streamer = Streamer(name="John", id="7").save(db)
team = Team(name="Warriors").save(db)

We will use these objects in the following examples.

Load nodes

There are many examples of when loading a node from the database may come in handy, but let's cover the two most common.

Finding node properties

Suppose you just have the id of the streamer and you want to know the streamer's name. You have to load that node from the database and check its name property.

loaded_streamer = Streamer(id="7").load(db=db)
print(loaded_streamer.name)

The code block above will print out the name of the stream whose id equals "7", that is, "John".

Creating relationship between existing nodes

Suppose you want to create a new relationship of type IS_PART_OF, between already saved streamer and team nodes. Then you first need to load them:

loaded_streamer = Stream(id="7").load(db=db)
loaded_team = Team(name="Warriors").load(db=db)

Since the id property is unique, the load() method will return a unique streamer found in the database. Similarly, since the name property is unique for each team, the load() method will return a unique team found in the database. When the given property is not unique, the load() method will return a list of matching results. Finally, you can create a relationship between loaded_streamer and loaded_team nodes:

is_part_of = IsPartOf(
                    _start_node_id=loaded_streamer._id, 
                    _end_node_id=loaded_team._id,
                    date="2021-04-26"
                ).save(db)

Load relationships

You can load a relationship from the database based on its start and end node. You have already created is_part_of relationship with streamer._id as _start_node_id and team._id as _end_node_id. Now you can load it from the database:

loaded_is_part_of = IsPartOf(
        _start_node_id=streamer._id,
        _end_node_id=team._id
    ).load(db)

It's easy to get its date property:

print(loaded_is_part_of.date)

The output of the above print is 2021-04-26.

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