Need to add sparse annotation support to model field

[tearsofphoenix]

Problem:

type ModelA struct {
	mongodm.DocumentBase `json:",inline" bson:",inline"`

	Target bson.ObjectId `json:"target" bson:"target"`
}

if create a ModelA but leave target field empty, when try to save the document, error occured:
ObjectIDs must be exactly 12 bytes long (got 0). So, maybe need add something like this:

Target bson.ObjectId `sparse:"true"`

?