Zero value of slices

[FcoManueel]

short version: It would be nice if null::text[] gets decoded as []string(nil) instead of []string{}

A column of array type, let say text[], can be mapped to a slice of strings []string. In such column we can store both a null array (null::text[]) or an empty array ('{}'::text[]).
Go has equivalents for both of them, a null array would be []string(nil) and an empty array would be []string{}.

Currently the library makes no difference between null and empty arrays, and maps both to []string{}. The issue with this is that the zero value of a slice is a nil slice, not an empty one, so if I have an structure like the following:

type Object struct{
    id               string
    stringSlice []string
}

and then I create a new Object in the following way:

a := Object{id: "1"}

then a.stringSlice will have a value of []string(nil), but if I save a to the db and then retrieve it, the stringSlice property gets decoded as []string{}. Therefore, if I compare my original struct and my retrieved struct, they are different.

Here's a playground with the mentioned example: http://play.golang.org/p/N4rf_47sL-

Same thing applies to the other supported slice types.

Thanks for such a great package!

[vmihailenco]

Hi,

Unfortunately I can't reproduce the issue:

package main

import (
    "fmt"

    "gopkg.in/pg.v3"
)

type Person struct {
    Emails []string
}

func main() {
    db := pg.Connect(&pg.Options{
        User: "postgres",
    })

    var person Person
    _, err := db.QueryOne(&person, `SELECT null::text[] AS emails`)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%#v\n", person)

    var emails []string
    _, err = db.QueryOne(pg.LoadInto(&emails), `SELECT null::text[] AS emails`)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%#v\n", emails)
}

main.Person{Emails:[]string(nil)}
[]string(nil)

And there is also a test for decoding nil slice: https://github.com/go-pg/pg/blob/master/types_test.go#L189

Can it be that you use old version of the package?

[FcoManueel]

I see. I assumed that what I was seeing was happening while decoding the slice, but now I see its when inserting the value.

Inside the method appendStringSlice there's this part:

if len(v) == 0 {
    return append(dst, `{}`...)
}

and, since both empty and nil slices have length 0, we will append {} to the query for both.

I confirmed this by adding at line 80 the following check:

if reflect.DeepEqual(src, []string(nil)) {
    return appendNull(dst)
}

... of course there must be better ways to do it 😄specially since the same is happening for the other types of slices.

After adding that code, my original and retrieved structs were the same.

[FcoManueel]

I'll paste here the code I used to reproduce my original use case, in case it's of any use:

First I created a table inside the test database

CREATE TABLE persons (
id text NOT NULL,
emails text[]
);

And this is my test code:

package main

import (
    "fmt"
    "gopkg.in/pg.v3"
)

type Person struct {
    ID     string
    Emails []string
}

type Persons[]*Person
func (contents *Persons) NewRecord() interface{} {
    content := &Person{}
    *contents = append(*contents, content)
    return content
}

func main() {
    db := pg.Connect(&pg.Options{
        User: "postgres",
        Database: "test",
    })
    db.Exec(`TRUNCATE TABLE persons;`)

    // Create and insert a person without emails
    var person1 = Person{ID: "1"}
    fmt.Printf("%#v\n", person1.Emails)
    _, err := db.ExecOne(`INSERT INTO persons(id, emails) VALUES (?id, ?emails);`, person1)

    // Get person from database
    var allPersons Persons
    _, err = db.Query(&allPersons, `SELECT id, emails FROM persons;`)
    if err != nil {
        panic(err)
    }

    if len(allPersons) != 1 {
        fmt.Println("len(allPersons) :", len(allPersons))
        panic("Expected only 1 person")
    }
    personFromDb := allPersons[0]
    fmt.Printf("%#v\n", personFromDb.Emails)
}

The obtained output was the following:

[]string(nil)
[]string{}

[vmihailenco]

Thanks for the report. Should be fixed in v3.3.14.

[FcoManueel]

Awesome, it works.
Thanks!

[vmihailenco]

You are welcome!