Mocking mgo chaining functions

While testing some of my codebase I found it useful to mock out objects from "gopkg.in/mgo.v2" with interfaces. I'm running into a problem where mgo.Query does not implement my interface Query.

import mgo "gopkg.in/mgo.v2"

type Collection interface {
	FindId(interface{}) Query
	Find(interface{}) Query
	UpdateId(interface{}, interface{}) error
	Update(interface{}, interface{}) error
	UpsertId(interface{}, interface{}) (interface{}, error)
	Insert(...interface{}) error
	RemoveId(interface{}) error
}

type Query interface {
	One(interface{}) error
	All(interface{}) error
	Select(interface{}) Query
}

var _ Query = (*mgo.Query)(nil)

The Query cast kicks up the error:

cannot use (*mgo.Query)(nil) (type *mgo.Query) as type Query in assignment:
    *mgo.Query does not implement Query (wrong type for Select method)
        have Select(interface {}) *mgo.Query
        want Select(interface {}) Query

Is this a problem with chain functions not being able to be defined in an interface? I'm not sure how to make a Select header that will match the mgo implementation.

The function signatures are different, which is why you're running into a compilation error. Your interface version of Select returns your Query type, while mgo's Select returns *mgo.Query, which is a different type. Even if that type does implement your interface, the function signatures are still different. You will need to add another layer to this where you are able to wrap the mgo package.

type MgoQuery struct {
    *mgo.Query
}

func (q *MgoQuery) Select(selector interface{}) Query {
    return q.Query.Select(selector)
}

...