[feature request] add shortcut method to get by primary key

[Tarjei Huse]

Hi, I often find myself querying for a single entity by an known primary key.

It would be super useful to have this as a separate method find(primaryKey).

[Artem Zakharchenko]

Hey, @tarjei.

Do I understand you correctly that you'd like to propose a shorthand for this:

const db = factory({
  user: { id: primaryKey(String) }
})

// For this particular query by the primary key ("id").
db.user.findOne({
  where: { id: { equals: 'abc-123' } }
})