feat: add query interface

[FLAGLORD]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Now gen.WithQueryInterface only generates interface for DO object. Considering that DO object uses context to initialize, most services would import Query instead of some DO object. In that case, it would be difficult to mock Query because Query is just a struct instead of interface. It would be easier to mock if we have a interface for Query.

User Case Description

[FLAGLORD]

#342

[jinzhu]

Can you add more details about why to mock the query interface?

Thank you.

[FLAGLORD]

I would add some explanations:
Assuming that I have a table Person, gen.WithQueryInterface will generate files following.
person.gen.go

// struct with do
type person struct{
	// personDo implements IPersonDO
	personDo personDo
	// ...
}

// interface for query
type IPersonDO interface {
	// ...
}

func(p *person) WithContext(ctx context.Context) IPersonDO{
	return p.personDo.WithContext(ctx)
}

// ...

gen.go

type Query struct {
	db *gorm.DB

	Person person
}

type queryCtx struct {
	Person IPersonDO
}

func (q *Query) WithContext(ctx context.Context) *queryCtx {
	// ...
}

If a ServiceA has a dependency on table Person, it's easy to write a constructor function like this:

func NewServiceA(personDO IPersonDO)*ServiceA{
	// ...
}

It's also easy to inject a mocked IPersonDO to write some test. (a mocked IPersonDO can be generated by mockery or something like that)

But if ServiceA is a more complicated service, it may use many tables. Thus it's easier to accept Query as a parameter for its constructor function.

But At this time, injecting a mocked interface is difficult. Query is a struct and its field Person doesn't implement IPersonDO. So we can't even set a mocked IPersonDO to the field Person of Query.

[stoffeastrom]

What about retrieving objects with primary key?

func PersonHandler(id uuid.UUID, q generatedpkg.IQuery) {
  p, err := q.PersonDo().Where(
    // How to retrive with primary key through interface?
    q.PersonDo().Fields().ID().Eq(id) // This will not work but would be nice to expose the fields as an interface as well
  ).First()
}