How to create a graph projection

As subgraphs are mainly used with Memgraph's query modules (graph algorithms), QueryBuilder's call() method enables specifying the subgraph to use with a certain algorithm.

To call a procedure named test_query_module with argument "arg", and run it on a subgraph containing only nodes with label :LABEL and their mutual relationships build the following query:

from gqlalchemy import QueryBuilder

label = "LABEL"

query_builder = QueryBuilder().call(procedure="test_query_module",
                                    arguments=("arg"), node_labels=label)

query_builder.execute()

The above code executes the following Cypher query:

MATCH p=(a)-->(b)
WHERE (a:LABEL)
AND (b:LABEL)
WITH project(p) AS graph
CALL test_query_module(graph, 'arg')

WHERE and AND clauses are used to allow for more generalization. To expand on this code you can use multiple relationship types and node labels. Node labels and relationship types can be passed as a single string, in which case that string is used for all labels or types. To specify different labels and types for entities on a path, you need to pass a list of lists, containing a list of labels for every node on a path, and likewise for relationships. You can use this as following:

node_labels = [["COMP", "DEVICE"], ["USER"], ["SERVICE", "GATEWAY"]]
relationship_types = [["OWNER", "RENTEE"], ["USES", "MAKES"]]
relationship_directions = [RelationshipDirection.LEFT, RelationshipDirection.RIGHT]
arguments = ("arg0", 5)

query_builder = QueryBuilder().call(procedure="test_query_module",
                                    arguments = arguments,
                                    node_labels=node_labels,
                                    relationship_types=relationship_types,
                                    relationship_directions=relationship_directions)

query_builder.execute()

The above code executes the following Cypher query:

MATCH p=(a)<-[:OWNER | :RENTEE]-(b)-[:USES | :MAKES]->(c)
WHERE (a:COMP or a:DEVICE)
AND (b:USER)
AND (c:SERVICE or c:GATEWAY)
WITH project(p) AS graph
CALL test_query_module(graph, "arg0", 5)

This query calls test_query_module on a subgraph containing all nodes labeled USER that have an outgoing relationship of types either OWNER or RENTEE towards nodes labeled COMP or DEVICE and also a relationship of type USES or MAKES towards nodes labeled SERVICE or GATEWAY.