序列化为JSON动态结构

Question

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.

Answer 1

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.