MikroORM does unnecessary JOINs when matching values on primary keys expliciting the PK column

[darkbasic]

@Entity()
export class Message extends BaseIdEntity {
  @ManyToMany({pivotTable: 'message_to_deletion'})
  deletions: Collection<User> = new Collection<User>(this);

em.find(Message, {
  group: groupId,
  deletions: 1,
});

The previous query matches a primary key on the M:N deletions field, without explicitly mentioning the PK column. This is the sql it generates:

select "e0".*, "e1"."message_id", "e1"."user_id"
from "message" as "e0"
left join "message_to_deletion" as "e1"
on "e0"."id" = "e1"."message_id"
where "e0"."group_id" = 1 and "e1"."user_id" = 1

In the following one, instead, we explictly specify the PK column (id):

em.find(Message, {
  group: groupId,
  deletions: {id: 1},
});

select "e0".*
from "message" as "e0"
left join "message_to_deletion" as "e2"
on "e0"."id" = "e2"."message_id"
left join "user" as "e1"
on "e2"."user_id" = "e1"."id"
where "e0"."group_id" = 1 and "e1"."id" = 1

As you can see in the generated sql it does an unnecessary JOIN on the user table. The JOIN is unnecessary because we are matching its primary key.

Expected behavior
Avoid a second JOIN when matching PKs.

Additional context
The expected behaviour is surely a desiderable one, but I do get why for semplicity this has been done: if you want to match a PK just use a scalar instead of explicitly specify the PK column.
The problem is that you cannot match null values unless you explicitely specify the PK column, see #548
So fixing this could help to achieve LEFT JOIN / IS NULL-like constraints (see https://explainextended.com/2009/09/15/not-in-vs-not-exists-vs-left-join-is-null-sql-server/) necessary to implement the following type of JOIN:

In fact the following snippet succeeds in implementing a LEFT JOIN / IS NULL-like constraint, except for the unnecessary JOIN which is the reason of this bug report:

em.find(Message, {
  group: groupId,
  $or: [{deletions: {id: null}}, {deletions: {$ne: 1}}],
});

select "e0".*
from "message" as "e0"
left join "message_to_deletion" as "e2"
on "e0"."id" = "e2"."message_id"
left join "user" as "e1"
on "e2"."user_id" = "e1"."id"
where "e0"."group_id" = 1 and ("e1"."id" is null or "e2"."user_id" != 1)

Versions

Dependency	Version
node	v13.14.0
typescript	3.8.3
mikro-orm	3.6.11
pg	8.0.3

[B4nan]

Closing as fixed in dev via 24511a6