Is there a way to filter by joined tables

[kbonevska]

Discussed in #1904

^{Originally posted by kbonevska August 4, 2021}
Hey, there guys, I've been using your project for some time and it seems amazing by now. Thank you for your effort.
I was wondering if I am missing something and there is actually a way to filter by joined tables. So far following the examples, it doesn`t seem possible by using Relation().

type Product struct {
	ID               int64
	Name             string
        Tags             []GrTag             `pg:",many2many:product_tags"`
}

In the following model, while selecting products with their tags, how can I sort by tag names, I tried something like

DB.Model(&p).Column("product.*").
Relation("Tags", func ( q *pg.Query)(*pg.Query, error){
	return q.Where("tag.name = 'tag 1'"), nil
})

but this will filter out just tags by the name, not products by the tag name