Serialize to JSON dynamic structure

All examples of working with JSON describe how to serialize to JSON simple or user types (like a struct).

But I have a different case: a) I don't know the fields of my type/object b) every object will have different types.

Here is my case in pseudocode:

while `select * from item` do
  while `select fieldname, fieldvalue from fields where fields.itemid = item.id` do
    ...

For each entity in my database I get field names and field values. In the result I need to get something like this:

{
  "item.field1": value,
  ...
  "item.fieldN": value,
  "custom_fields": {
     "fields.field1": value,
      ...
     "fields.fieldK": value
  }
}

What is the best way to do it in Go? Is there any useful libraries or functions in standard library ?

Update: The source of data is the database. In the result i need to get JSON as string to POST it to external web service. So, the program just read data from database and make POST requests to REST service.

What exactly is your target type supposed to be? It can't be a struct since you do not know the fields beforehand.

The only fitting type to me seems to be a map of type map[string]interface{}: with it any nested structure can be achieved:

a := map[string]interface{}{
	"item.field1": "val1",
	"item.field2": "val2",
	"item.fieldN": "valN",
	"custom_fields": map[string]interface{}{
		"fields.field1": "cval1",
		"fields.field2": "cval2",
	},
}
b, err := json.Marshal(a)

See playground sample here.

Filling this structure from a database as you hinted at should probably be a custom script (not using json).

Note: custom_fields can also be of other types depending on what type the value column is in the database. If the value column is a string use map[string]string.