Scan a PostgreSQL field (of ARRAY type) into a slice of Go structs - sql

Question

Let's say I have:
type User struct {
ID int64 `json:"id"
Posts []Post `json:"posts"
}
type Post struct {
ID int64 `json:"id"
Text string `json:"text"
}
The SQL query:
WITH temp AS (SELECT u.id AS user_id, p.id AS post_id, p.text AS post_text FROM users u JOIN posts p ON u.id=p.user_id)
SELECT user_id, ARRAY_AGG(ARRAY[post_id::text, post_text])
FROM temp
GROUP BY user_id
)
What I want is to scan rows from the query above into a slice of User objects:
import (
"context"
"fmt"
"github.com/jackc/pgx/v4/pgxpool"
"github.com/lib/pq"
)
var out []User
rows, _ := client.Query(context.Background(), query) // No error handling for brevity
for rows.Next() {
var u User
if err := rows.Scan(&u.ID, pq.Array(&u.Posts)); err != nil {
return
}
out = append(out, u)
}
Pretty much expectedly, the code above fails with:
pq: cannot convert ARRAY[4][2] to StringArray
This makes sense, but is there a way to read the SQL output into my slice of users?

Answer 1

Scanning of multi-dimensional arrays of arbitrary types, like structs, is not supported by lib/pq. If you want to scan such an array you'll have to parse and decode it yourself in a custom sql.Scanner implementation.
For example:
type PostList []Post
func (ls *PostList) Scan(src any) error {
var data []byte
switch v := src.(type) {
case string:
data = []byte(v)
case []byte:
data = v
}
// The data var holds the multi-dimensional array value,
// something like: {{"1","foo"}, {"2","bar"}, ...}
// The above example is easy to parse but too simplistic,
// the array is likely to be more complex and therefore
// harder to parse, but not at all impossible if that's
// what you want.
return nil
}
If you want to learn more about the PostgreSQL array representation syntax, see:
Array Input and Output Syntax
An approach that does not require you to implement a parser for PostgreSQL arrays would be to build and pass JSON objects, instead of PostgreSQL arrays, to array_agg. The result of that would be a one-dimensional array with jsonb as the element type.
SELECT user_id, array_agg(jsonb_build_object('id', post_id, 'text', post_text))
FROM temp
GROUP BY user_id
Then the implementation of the custom sql.Scanner just needs to delegate to lib/pq.GenericArray and another, element-specific sql.Scanner, would delegate to encoding/json.
type PostList []Post
func (ls *PostList) Scan(src any) error {
return pq.GenericArray{ls}.Scan(src)
}
func (p *Post) Scan(src any) error {
var data []byte
switch v := src.(type) {
case string:
data = []byte(v)
case []byte:
data = v
}
return json.Unmarshal(data, p)
}
type User struct {
ID int64 `json:"id"`
Posts PostList `json:"posts"`
}