How to refactor this code to retrieve custom (virtual) columns from DB with one query only using an ORM

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How to refactor this code to retrieve custom (virtual) columns from DB with one query only using an ORM

I’m studying Golang trying to build an application.

I have written many models:

  • Player

  • Book

  • Score

  • Referee

  • Life

  • Team

  • Doc

  • Family

and so on…

A model is like this:


type Player struct {

  id   int

  name string

}

I’m using an ORM: Go-PG (https://github.com/go-pg/pg) and before I also tried GORM (https://github.com/jinzhu/gorm).

I’m using it like this:


db := database.New(config)

defer db.Close()

var players []Player

error := db.Model(&players).Select()

// handle error

and it works.

The SQL query is:


SELECT * FROM players

with these rows:

+--------+----------+
| id     | name     |
+--------+----------+
| 1      | John     |
+--------+----------+
| 2      | Mike     |
+--------+----------+
| 3      | Bob      |
+--------+----------+

Now I need to query from DB one or more “virtual” columns like this:


'mysqlcalculation' AS mycalculation

so I tried to use:


query.Column("*").ColumnExpr("'mysqlcalculation' AS mycalculation")

which generates a correct SQL query:


SELECT *, 'mysqlcalculation' AS mycalculation FROM players

with these rows:

+--------+----------+------------------+
| id     | name     | mycalculation    |
+--------+----------+------------------+
| 1      | John     | mysqlcalculation |
+--------+----------+------------------+
| 2      | Mike     | mysqlcalculation |
+--------+----------+------------------+
| 3      | Bob      | mysqlcalculation |
+--------+----------+------------------+

I’m doing this because I am interested in the result in the mycalculation column which is much more convenient to calculate in the database than in Go; it can be JSON or string. Simply data.

Now my ORM panic with:

PNC error="pg: can't find column=mycalculation in model=Player (try discard_unknown_columns)"

I can understand that because now go-pg doesn’t know how to bind data, so now I’m using:

func FindAllPlayers(ctx context.Context, pagination models.Pagination) ([]*models.Player, *string, error) {

  var cursor *string

  var playerWithCalculation []struct {

    Mycalculation  string

    models.Player

  }

  var players []Player

  // use pagination for changing query settings

  error := db.Model(&playerWithCalculation).Column("*").ColumnExpr("'mysqlcalculation' AS mycalculation").Select()

  // handle error

  cursor := playerWithCalculation[0].Mycalculation

  for i := range playerWithCalculation {

    players = append(players, &playerWithCalculation[i].Player)

  }

return players, cursor, nil

QUESTION

I think this is wasteful and expensive. Many allocations and loops.

And besides, I have several models and resolvers and methods!

What can I do?

How can I improve this code and my architecture?

I think a possible solution is to run two queries, perhaps in two different goroutines.

But I don’t want to run two queries for this.