Postgres array of Golang structs - postgresql

Question

I have the following Go struct:
type Bar struct {
Stuff string `db:"stuff"`
Other string `db:"other"`
}
type Foo struct {
ID int `db:"id"`
Bars []*Bar `db:"bars"`
}
So Foo contains a slice of Bar pointers. I also have the following tables in Postgres:
CREATE TABLE foo (
id INT
)
CREATE TABLE bar (
id INT,
stuff VARCHAR,
other VARCHAR,
trash VARCHAR
)
I want to LEFT JOIN on table bar and aggregate it as an array to be stored in the struct Foo. I've tried:
SELECT f.*,
ARRAY_AGG(b.stuff, b.other) AS bars
FROM foo f
LEFT JOIN bar b
ON f.id = b.id
WHERE f.id = $1
GROUP BY f.id
But it looks like the ARRAY_AGG function signature is incorrect (function array_agg(character varying, character varying) does not exist). Is there a way to do this without making a separate query to bar?

Answer 1

It looks like what you want is for bars to be an array of bar objects to match your Go types. To do this, you should use JSON_AGG rather than ARRAY_AGG since ARRAY_AGG only works on single columns and would produce in this case an array of type text (TEXT[]). JSON_AGG, on the other hand, creates an array of json objects. You can combine this with JSON_BUILD_OBJECT to select only the columns you want.
Here's an example:
SELECT f.*,
JSON_AGG(JSON_BUILD_OBJECT('stuff', b.stuff, 'other', b.other)) AS bars
FROM foo f
LEFT JOIN bar b
ON f.id = b.id
WHERE f.id = $1
GROUP BY f.id
Then you'll have to handle unmarshaling the json in Go, but other than that you should be good to go.
Note also that Go will ignore unused keys for you when unmarshaling json to a struct, so you can simplify the query by just selecting all fields on the bar table if you want. Like so:
SELECT f.*,
JSON_AGG(TO_JSON(b.*)) AS bars -- or JSON_AGG(b.*)
FROM foo f
LEFT JOIN bar b
ON f.id = b.id
WHERE f.id = $1
GROUP BY f.id
If you want to also handle cases where there are no entries in bar for a record in foo, you can use:
SELECT f.*,
COALESCE(
JSON_AGG(TO_JSON(b.*)) FILTER (WHERE b.id IS NOT NULL),
'[]'::JSON
) AS bars
FROM foo f
LEFT JOIN bar b
ON f.id = b.id
WHERE f.id = $1
GROUP BY f.id
Without the FILTER, you'll get [NULL] for rows in foo that have no corresponding rows in bar, and the FILTER gives you just NULL instead, then just use COALESCE to convert to an empty json array.

Answer 2

As you already know array_agg takes a single argument and returns an array of the type of the argument. So, if you want all of a row's columns to be included in the array's elements you can just pass in the row reference directly, e.g.:
SELECT array_agg(b) FROM b
If, however, you only want to include specific columns in the array's elements you can use the ROW constructor, e.g.:
SELECT array_agg(ROW(b.stuff, b.other)) FROM b
Go's standard library provides out-of-the-box support for scanning only scalar values. For scanning more complex values like arbitrary objects and arrays one has to either look for 3rd party solutions, or implement their own sql.Scanner.
To be able to implement your own sql.Scanner and properly parse a postgres array of rows you first need to know what format postgres uses to output the value, you can find this out by using psql and some queries directly:
-- simple values
SELECT ARRAY[ROW(123,'foo'),ROW(456,'bar')];
-- output: {"(123,foo)","(456,bar)"}
-- not so simple values
SELECT ARRAY[ROW(1,'a b'),ROW(2,'a,b'),ROW(3,'a",b'),ROW(4,'(a,b)'),ROW(5,'"','""')];
-- output: {"(1,\"a b\")","(2,\"a,b\")","(3,\"a\"\",b\")","(4,\"(a,b)\")","(5,\"\"\"\",\"\"\"\"\"\")"}
As you can see this can get pretty hairy but nevertheless it's parseable, the syntax looks to be something like this:
{"(column_value[, ...])"[, ...]}
where column_value is either an unquoted value, or a quoted value with escaped double quotes, and such a quoted value itself can contain escaped double quotes but only in twos, i.e. a single escaped double quote will not occur inside the column_value. So a rough and incomplete implementation of the parser might look something like this:
NOTE: there may be other syntax rules, that I do not know of, that need to be taken into consideration during parsing. In addition to that the code below doesn't handle NULLs properly.
func parseRowArray(a []byte) (out [][]string) {
a = a[1 : len(a)-1] // drop surrounding curlies
for i := 0; i < len(a); i++ {
if a[i] == '"' { // start of row element
row := []string{}
i += 2 // skip over current '"' and the following '('
for j := i; j < len(a); j++ {
if a[j] == '\\' && a[j+1] == '"' { // start of quoted column value
var col string // column value
j += 2 // skip over current '\' and following '"'
for k := j; k < len(a); k++ {
if a[k] == '\\' && a[k+1] == '"' { // end of quoted column, maybe
if a[k+2] == '\\' && a[k+3] == '"' { // nope, just escaped quote
col += string(a[j:k]) + `"`
k += 3 // skip over `\"\` (the k++ in the for statement will skip over the `"`)
j = k + 1 // skip over `\"\"`
continue // go to k loop
} else { // yes, end of quoted column
col += string(a[j:k])
row = append(row, col)
j = k + 2 // skip over `\"`
break // go back to j loop
}
}
}
if a[j] == ')' { // row end
out = append(out, row)
i = j + 1 // advance i to j's position and skip the potential ','
break // go to back i loop
}
} else { // assume non quoted column value
for k := j; k < len(a); k++ {
if a[k] == ',' || a[k] == ')' { // column value end
col := string(a[j:k])
row = append(row, col)
j = k // advance j to k's position
break // go back to j loop
}
}
if a[j] == ')' { // row end
out = append(out, row)
i = j + 1 // advance i to j's position and skip the potential ','
break // go to back i loop
}
}
}
}
}
return out
}
Try it on playground.
With something like that you can then implement an sql.Scanner for your Go slice of bars.
type BarList []*Bar
func (ls *BarList) Scan(src interface{}) error {
switch data := src.(type) {
case []byte:
a := praseRowArray(data)
res := make(BarList, len(a))
for i := 0; i < len(a); i++ {
bar := new(Bar)
// Here i'm assuming the parser produced a slice of at least two
// strings, if there are cases where this may not be the true you
// should add proper length checks to avoid unnecessary panics.
bar.Stuff = a[i][0]
bar.Other = a[i][1]
res[i] = bar
}
*ls = res
}
return nil
}
Now if you change the type of the Bars field in the Foo type from []*Bar to BarList you'll be able to directly pass in a pointer of the field to a (*sql.Row|*sql.Rows).Scan call:
rows.Scan(&f.Bars)
If you don't want to change the field's type you can still make it work by converting the pointer just when it's being passed to the Scan method:
rows.Scan((*BarList)(&f.Bars))
JSON
An sql.Scanner implementation for the json solution suggested by Henry Woody would look something like this:
type BarList []*Bar
func (ls *BarList) Scan(src interface{}) error {
if b, ok := src.([]byte); ok {
return json.Unmarshal(b, ls)
}
return nil
}