Script field using expression - Only the member variable [value] or member methods may be accessed

Question

I am trying to use script fields in my query. I enabled sandbox scripting, and trying to use an expression to calculate a new field.

The thing is I get the following error:

{
   "type": "expression_script_compilation_exception",
   "reason": "Only the member variable [value] or member methods may be accessed on a field when not accessing the field directly"
}

It seems that only "value" is accessible. what am I missing here?

When running the following query:

{
  "query": {
    "match_all": {}
  },
  "script_fields" : {
    "field1" : {
        "script" : {
            "lang": "expression",
            "inline": "doc['about.hobbies'].empty"
        }
    }
  }
}

Mapping:

{
  "my_index": {
  "mappings": {
     "type": {
        "properties": {
           "about": {
              "properties": {
                 "hobbies": {
                    "type": "string",
                    "analyzer": "lowercase"
                 }
              }
           }
        }  
     }
   }
}

Little explanation: I have a field which can contain list of string values.

"hobbies": ["a","b",c"]

and it can be also empty. I want to have a script field of type boolean that will have the value true when the list is not empty, and false when the list is empty.

Update: Reading some more, I encounter this documentation on lucene expressions scripts

There are a few limitations relative to other script languages:

• Only numeric fields may be accessed
• Stored fields are not available
• If a field is sparse (only some documents contain a value), documents missing the field will have a value of 0

My field is of type String, it might be the problem? If it is, is there any other way to use script fields based on string fields? maybe using groovy?

Answer 1

I think the problem is that the field is a nested object, if I read the docs correctly then doc['field'] only supports simple term fields.

Note, however, that the doc[...] notation only allows for simple valued fields (can’t return a json object from it) and makes sense only on non-analyzed or single term based fields.

However using _source has worked for me

   "script_fields" : {
      "field1" : {
           "script" : "_source.about.hobbies.size() > 0"
       }
    },