Using go-pg to retrieve virtual columns from Postgres - database

Question

I'm using go-pg (https://github.com/go-pg/pg) and this code:
type Book struct {
id int
name string
}
var books []Book
err := db.Model(&books).Select()
and everything works good but I need to add a "virtual" column like this:
concat ('info:', 'id:', id, '...') AS info
and I tried to use:
query.ColumnExpr("concat ('info:', 'id:', id, '...') AS info")
but:
go-pg complains with: error="pg: can't find column=info in model=Book (try discard_unknown_columns)"
go-pg doesn't include anymore columns id and name in query: concat... ONLY!
I can understand that because now go-pg doesn't know how to bind data, but I really need that string which I can retrieve from DB only.
Is there a way?
Can I use a custom type like this below?
type CustomBook struct {
Info string
Book
}
Does this make sense?

Answer 1

this approach could work for you:
type Book struct {
ID int
Name string
Info string `pg:"-"`
}
...
db.Model(&books).ColumnExpr("book.*").ColumnExpr("CONCAT('id:', id, 'name:', name) AS info").Select()
pg:"-" ignores the struct field and it is not created nor it produces any errors
this ignored column is documented here: https://pg.uptrace.dev/models/
another approach, depending on your requirements could be like this:
var r []struct {
Name string
Info string
}
db.Model((*Book)(nil)).Column("name").ColumnExpr("CONCAT('id:', id, 'name:', name) AS info").Select(&r)
this second one is documented here: https://pg.uptrace.dev/queries/