WIP: feat: add support for many-to-many joins

[joelmukuthu]

Usage:

class User extends Model {}
User.table = 'user';
User.fields = {
  id: 'integer'
};

class Group extends Model {}
Group.table = 'user';
Group.fields = {
  id: 'integer'
};

class GroupMembership extends Model {}
GroupMembership.table = 'group_membership';
GroupMembership.fields = {
  userId: { type: 'integer', references: User.fields.id },
  groupId: { type: 'integer', references: Group.fields.id }
};

User.fetch({
  leftJoin: Group.query
    .via(GroupMembership)
    .as('groups')
});

which yields SQL similar to:

SELECT * FROM  "user" "u" LEFT JOIN "group_membership" "m" ON "m"."user_id" = "u"."id" LEFT JOIN "group" "g" ON "m"."group_id" = "g"."id"

and output such as:

const users = [
  new User({ 
    id: 1, 
    groups: [new Group({ id: 1 })]
  })
];

Closes knorm/knorm#143

[joelmukuthu]

Instead of a via option, it makes more sense to go with an api resembling the current api around joins:

User.fetch({
  leftJoin: GroupMembership.query
    .asJoinQuery(true) // tells @knorm/relations to not include this join in the query results
    .join(Group.query.as('groups'))
});

[coveralls]

Pull Request Test Coverage Report for Build 299

• 82 of 82 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 99.13%

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Totals
Change from base Build 281:	0.2%
Covered Lines:	227
Relevant Lines:	227

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💛 - Coveralls

[boh1996]

Wouldn't it make sense to return an empty array here?

[joelmukuthu]

Yeah, that's a bug: #8 I intend to fix it in v2 (breaking change)