Subquery needs to return more than 1 row?

Using Django Using the ORM
15

Model code:

# part.py

class Part(models.Model):
    id = HashidSaltedAutoField(primary_key=True)
    is_active = models.BooleanField(default=True)
    components = models.ManyToManyField("Component", related_name="parts")

# component.py

class Component(models.Model):
    id = HashidSaltedAutoField(primary_key=True)
    engine_machine = models.ForeignKey(
        "Machine",
        null=True,
        blank=True,
        related_name="machine_engine",
        on_delete=models.SET_NULL,
    )
    transmission_machine = models.ForeignKey(
        "Machine",
        null=True,
        blank=True,
        related_name="machine_transmission",
        on_delete=models.SET_NULL,
    )

# machine.py

class Machine(models.Model):
    id = HashidSaltedAutoField(primary_key=True)

Generated SQL using latest iteration of the query:

SELECT
	"machine"."id",
	COALESCE((
	SELECT
		DISTINCT COUNT(U0."id") AS "count"
	FROM
		"part" U0
	INNER JOIN "part_components" U1 ON
		(U0."id" = U1."part_id")
	INNER JOIN "component" U2 ON
		(U1."component_id" = U2."id")
	WHERE
		((U2."machine_transmisssion_id" = "machine"."id"
			OR U2."machine_engine_id" = "machine"."id")
		AND U0."is_active")
	GROUP BY
		U0."id"
	LIMIT 1),
	0) AS "num_parts"
FROM
	"machine"

Results from test:

# |------------+---------------------+----------------+--------------+--------------|
# | machine    | components          | parts          | expected     | output       |
# |            |                     |                | num_parts    | num_parts    |
# |------------+---------------------+----------------+--------------+--------------|
# | r1         | f1                  | ps1            | 2            | 1            |
# |            | f2                  | ps1            |              |              |
# |            |                     | ps2            |              |              |
# |------------+---------------------+----------------+--------------+--------------|