`Subgraph.__db_create__` doesn't create multiple relationships between the same nodes

[alfredo-f]

Issue

Description

from py2neo import Graph, Node
graph = Graph(NEO4J_URI, user=NEO4J_USER, password=NEO4J_PASSWORD)
node_a = Node("Node", name="A")
node_b = Node("Node", name="B")

graph_tx = graph.begin()
for node in (node_a, node_b):
    graph_tx.create(node)
graph.commit(graph_tx)

relationship_1 = Relationship(node_a, "REL", node_b, id_overwritten=6)
relationship_2 = Relationship(node_a, "REL", node_b, id_overwritten=7)

graph_tx = graph.begin()
for node in (node_a, node_b):
    graph_tx.create(node)
graph.commit(graph_tx)

graph_tx = graph.begin()
for relationship in (relationship_1, relationship_2):
    graph_tx.create(relationship)
graph.commit(graph_tx)

Expected behavior

Given that relationship_2 has a different id_overwritten, a new relationship should be created.

(A) -[REL {"id_overwritten": 6}]-> (B)
(A) -[REL {"id_overwritten": 7}]-> (B)

Actual behavior

relationship_1 is replaced with relationship_2

(A) -[REL {"id_overwritten": 7}]-> (B)

Cause

This is due to the fact that Subgraph.__db_create__ has no option for CREATEing instead of MERGEing

https://github.com/py2neo-org/py2neo/blob/2e46bbf4d622f53282e796ffc521fc4bc6d0b60d/py2neo/data.py#L209

>>> print(pq[0])

UNWIND $data AS r
MATCH (a) WHERE id(a) = r[0]
MATCH (b) WHERE id(b) = r[2]
MERGE (a)-[_:REL]->(b)
SET _ += r[1]

Workaround

...
pq = unwind_merge_relationships_query(data, r_type)

import re

# Pardon my regex
pq = (
    re.sub(
        (
            r"(.*)?"
            r"(MERGE )(\(a\)-\[_:\w+\]->\(b\))"
            r"(.*)"
        ),
        (
            r"\g<1>"
            r"CREATE "
            r"\g<3>"
            r"\g<4>"
        ),
        pq[0],
    ),
    *pq[1:],
)
...

so that

>>> print(pq[0])

UNWIND $data AS r
MATCH (a) WHERE id(a) = r[0]
MATCH (b) WHERE id(b) = r[2]
CREATE (a)-[_:REL]->(b)
SET _ += r[1]