Implement Scan interface to read json array into map

Question

I am fetching a JSON array from PostgreSQL, and I would like to read it into a map. I am able to Unmarshal the values into a []string slice, but what I actually want is a map[string]bool.

I've written a custom type for the column with a Scan interface that converts the JSON array into a slice of strings first, then reads each string into the custom map type as keys.

type custMap map[string]bool

func (m *custMap) Scan(src interface{}) error {
  b, ok := src.([]byte)
  if !ok {
    return error(errors.New("Error Scanning Array"))
  }

  s := make([]string, 0)
  json.Unmarshal(b, &s)
  for _, v := range s {
    (*m)[v] = true
  }

  return nil
}


type data struct {
  vals custMap `json: "vals"`
}

The query I am trying to scan returns a row with a column vals which is a JSON array: ["some", "arr", "vals"], where the custom type is used like so:

var d models.data
sqlDB.QueryRow().Scan(&d.vals)

My expected output is a struct with the following shape

{ vals: map[string]bool { "some": true, "arr": true, "vals": true }

This compiles fine, but my code panics with "assignment to entry in nil map"

How can I fix my Scan function? Is it even possible to do this with a map type?

Please provide line numbers of the source and the error message. I suspect the error happens on the line (*m)[v] = true but that is just my suspicion. — typetetris
– typetetris, Commented Jun 2, 2017 at 7:36
Yes! that is where i'm getting 'assignment to entry in nil map' — minocys
– minocys, Commented Jun 2, 2017 at 7:50
The specific error thrown is 0http: panic serving [::1]:59847: assignment to entry in nil map. I know it's that line because for my first try at this I did not use a pointer there: m[v] = true Which resulted in this compile error: invalid operation: m[v] (type *SCMap does not support indexing) — minocys
– minocys, Commented Jun 2, 2017 at 7:51

typetetris · Accepted Answer · 2017-06-02 07:55:20Z

2

You are calling your method Scan of type *custMap on a unitialised map. Initialize d.vals either like

d.vals = custMap{}

or

d.vals = make(custMap)

Other answers already provide an explanation.

answered Jun 2, 2017 at 7:55

typetetris

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Comments

peterSO · Accepted Answer · 2017-06-02 07:51:27Z

The Go Programming Language Specification

Map types

A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type. The value of an uninitialized map is nil.

A new, empty map value is made using the built-in function make, which takes the map type and an optional capacity hint as arguments:
make(map[string]int)
make(map[string]int, 100)
The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. A nil map is equivalent to an empty map except that no elements may be added.

I don't see a make to initialize your map: "A nil map is equivalent to an empty map except that no elements may be added."

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