Enforcing minimum relationships

[ErisDS]

Normally:

• hasOne = has 0 or 1 child
• hasMany = has 0, 1, or many children
• belongsToMany = belongs to 0, 1 or many parents

However in some cases you might want more explicit relationships like:

• hasExactlyOne = always has 1 child
• hasAtLeastOne = has 1 or many children
• belongsToAtLeastOne = belongs to 1 or many parents

I can imagine in some cases there are even more complex enforcements, like having exactly X relations.

Bookshelf doesn't provide any of this behaviour, but bookshelf relations has the ability, I think, to enforce this kind of rule and abstract away the logic.

In Ghost we already have a use case for hasAtLeastOne with authors and potentially a case for hasExactlyOne for dividing the post table up behind the scenes to improve performance.

We maybe should add a way to configure these extra constraints in bookshelf-relations somehow?