Fields must be an object got – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 7.15-7.15

Briefly, this error occurs when Elasticsearch expects an object for the [fields] parameter, but receives a different data type. This usually happens when incorrect data types are used in the request body of a search query. To resolve this, ensure that the [fields] parameter is an object, not a string or an array. Check your query syntax and structure, and correct any discrepancies. Also, ensure that the data you’re passing matches the expected data type.

This guide will help you check for common problems that cause the log ” [fields] must be an object; got ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index.

 
 

index

Overview

In Elasticsearch, an index (plural: indices) contains a schema and can have one or more shards and replicas. An Elasticsearch index is divided into shards and each shard is an instance of a Lucene index.

Indices are used to store the documents in dedicated data structures corresponding to the data type of fields. For example, text fields are stored inside an inverted index whereas numeric and geo fields are stored inside BKD trees.

Examples

Create index

The following example is based on Elasticsearch version 5.x onwards. An index with two shards, each having one replica will be created with the name test_index1

PUT /test_index1?pretty
{
    "settings" : {
        "number_of_shards" : 2,
        "number_of_replicas" : 1
    },
    "mappings" : {
        "properties" : {
            "tags" : { "type" : "keyword" },
            "updated_at" : { "type" : "date" }
        }
    }
}

List indices

All the index names and their basic information can be retrieved using the following command:

GET _cat/indices?v

Index a document

Let’s add a document in the index with the command below:

PUT test_index1/_doc/1
{
  "tags": [
    "opster",
    "elasticsearch"
  ],
  "date": "01-01-2020"
}

Query an index

GET test_index1/_search
{
  "query": {
    "match_all": {}
  }
}

Query multiple indices

It is possible to search multiple indices with a single request. If it is a raw HTTP request, index names should be sent in comma-separated format, as shown in the example below, and in the case of a query via a programming language client such as python or Java, index names are to be sent in a list format.

GET test_index1,test_index2/_search

Delete indices

DELETE test_index1

Common problems

  • It is good practice to define the settings and mapping of an Index wherever possible because if this is not done, Elasticsearch tries to automatically guess the data type of fields at the time of indexing. This automatic process may have disadvantages, such as mapping conflicts, duplicate data and incorrect data types being set in the index. If the fields are not known in advance, it’s better to use dynamic index templates.
  • Elasticsearch supports wildcard patterns in Index names, which sometimes aids with querying multiple indices, but can also be very destructive too. For example, It is possible to delete all the indices in a single command using the following commands:
DELETE /*

To disable this, you can add the following lines in the elasticsearch.yml:

action.destructive_requires_name: true
 
 

 
 

Log Context

Log “[fields] must be an object; got” class name is CompositeRuntimeField.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 return builder;
 }  private static Map parseFields(String name; Object fieldsObject) {
 if (fieldsObject instanceof Map == false) {
 throw new MapperParsingException("[fields] must be an object; got " + fieldsObject.getClass().getSimpleName() +
 "[" + fieldsObject + "] for field [" + name +"]");
 }
 @SuppressWarnings("unchecked")
 Map fields = (Map) fieldsObject;
 return fields;

 

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