Example for builder.Lookup with SimpleAggregate please

[suntong]

Is your feature request related to a problem? Please describe.

How to use builder.Lookup with SimpleAggregate?

Describe the solution you'd like

Please add one more example using SimpleAggregate with builder.Lookup into
https://github.com/Kamva/mgm/blob/master/examples/aggregate/aggregate.go

Describe alternatives you've considered
Additional context

I tried it myself, adding the following to the top of func lookup(), after:

mgm/examples/aggregate/aggregate.go

Lines 24 to 27 in aff6e8d

	func lookup() error {
	seed()
	defer delSeededData()

{
	authorCollName := mgm.Coll(&author{}).Name()
	result := []book2{}
	_ := mgm.Coll(&book{}).SimpleAggregate(&result, builder.Lookup(authorCollName, "auth_id", "_id", "author"))
	fmt.Printf("1] %+v\n", result)
}

after defining book2 as:

type book2 struct {
	book
	author []author
}

However, what get printed was:

1] [{book:{DefaultModel:{IDField:{ID:ObjectID("000000000000000000000000")} DateFields:{CreatedAt:0001-01-01 00:00:00 +0000 UTC UpdatedAt:0001-01-01 00:00:00 +0000 UTC}} Name: Pages:0 AuthorID:ObjectID("000000000000000000000000")} author:[]}]

I.e., the lookup field author was empty, and so is the returned book, but the SimpleAggregate didn't return any error (I checked).

Please help.