Re-design the "Relation" class

[kettanaito]

Breaking changes

• Relational properties can no longer be updated using a compatible plain object. Only actual relations can be used to update relations.

Changes

• The Relation class now has distinct methods to initialize the relation and apply the property proxy.
• Scopes all the info needed for a relation in its class.
• Introduces the concepts of "source" (an entity that has a relation) and "target" (an entity that is being referenced).
• unique is not called a relation attribute.
• Moves unique relational properties tests to the respective relation kind tests (no longer a separate unique.test.ts).

Bug fixes

• Fixes an issue that prevented an update of a property if it wasn't specified when the entity was created.
• Fixes an issue that broke the querying by an entity's property if that property has no value.

db.user.create({ id: 'abc-123' })

// The following query would throw.
db.user.findFirst({ where: { anotherProperty: { equals: 'value' } } })

• Fixes an issue that would throw an exception when validating a unique relational value if that value is currently assigned to the entity that's being operated on (must exclude the current entity from the validation query).

GitHub

• Closes Re-design relational properties #127

Roadmap

• Cover the Relation class with unit tests.
• Review the new Relation class method names: apply and resolveWith.
• Resolve all @fixme comments.