Field mappings and transformations using Azure Search indexers

When using Azure Search indexers, you sometimes find that the input data doesn't quite match the schema of your target index. In those cases, you can use field mappings to reshape your data during the indexing process.

Some situations where field mappings are useful:

  • Your data source has a field named _id, but Azure Search doesn't allow field names that start with an underscore. A field mapping lets you effectively rename a field.
  • You want to populate several fields in the index from the same data source data. For example, you might want to apply different analyzers to those fields.
  • You want to populate an index field with data from more than one data source, and the data sources each use different field names.
  • You need to Base64 encode or decode your data. Field mappings support several mapping functions, including functions for Base64 encoding and decoding.


The field mapping feature of Azure Search indexers provides a simple way to map data fields to index fields, with a few options for data conversion. More complex data might require pre-processing to reshape it into a form that's easy to index.

Microsoft Azure Data Factory is a powerful cloud-based solution for importing and transforming data. You can also write code to transform source data before indexing. For code examples, see Model relational data and Model multilevel facets.

Set up field mappings

A field mapping consists of three parts:

  1. A sourceFieldName, which represents a field in your data source. This property is required.
  2. An optional targetFieldName, which represents a field in your search index. If omitted, the same name as in the data source is used.
  3. An optional mappingFunction, which can transform your data using one of several predefined functions. The full list of functions is below.

Field mappings are added to the fieldMappings array of the indexer definition.

Map fields using the REST API

You can add field mappings when creating a new indexer using the Create Indexer API request. You can manage the field mappings of an existing indexer using the Update Indexer API request.

For example, here's how to map a source field to a target field with a different name:

PUT https://[service name][api-version]
Content-Type: application/json
api-key: [admin key]
    "dataSourceName" : "mydatasource",
    "targetIndexName" : "myindex",
    "fieldMappings" : [ { "sourceFieldName" : "_id", "targetFieldName" : "id" } ]

A source field can be referenced in multiple field mappings. The following example shows how to "fork" a field, copying the same source field to two different index fields:

"fieldMappings" : [
    { "sourceFieldName" : "text", "targetFieldName" : "textStandardEnglishAnalyzer" },
    { "sourceFieldName" : "text", "targetFieldName" : "textSoundexAnalyzer" }


Azure Search uses case-insensitive comparison to resolve the field and function names in field mappings. This is convenient (you don't have to get all the casing right), but it means that your data source or index cannot have fields that differ only by case.

Map fields using the .NET SDK

You define field mappings in the .NET SDK using the FieldMapping class, which has the properties SourceFieldName and TargetFieldName, and an optional MappingFunction reference.

You can specify field mappings when constructing the indexer, or later by directly setting the Indexer.FieldMappings property.

The following C# example sets the field mappings when constructing an indexer.

  List<FieldMapping> map = new List<FieldMapping> {
    // removes a leading underscore from a field name
    new FieldMapping("_custId", "custId"),
    // URL-encodes a field for use as the index key
    new FieldMapping("docPath", "docId", FieldMappingFunction.Base64Encode() )

  Indexer sqlIndexer = new Indexer(
    name: "azure-sql-indexer",
    dataSourceName: sqlDataSource.Name,
    targetIndexName: index.Name,
    fieldMappings: map,
    schedule: new IndexingSchedule(TimeSpan.FromDays(1)));

  await searchService.Indexers.CreateOrUpdateAsync(indexer);

Field mapping functions

A field mapping function transforms the contents of a field before it's stored in the index. The following mapping functions are currently supported:

base64Encode function

Performs URL-safe Base64 encoding of the input string. Assumes that the input is UTF-8 encoded.

Example - document key lookup

Only URL-safe characters can appear in an Azure Search document key (because customers must be able to address the document using the Lookup API ). If the source field for your key contains URL-unsafe characters, you can use the base64Encode function to convert it at indexing time.

When you retrieve the encoded key at search time, you can then use the base64Decode function to get the original key value, and use that to retrieve the source document.

"fieldMappings" : [
    "sourceFieldName" : "SourceKey",
    "targetFieldName" : "IndexKey",
    "mappingFunction" : {
      "name" : "base64Encode",
      "parameters" : { "useHttpServerUtilityUrlTokenEncode" : false }

If you don't include a parameters property for your mapping function, it defaults to the value {"useHttpServerUtilityUrlTokenEncode" : true}.

Azure Search supports two different Base64 encodings. You should use the same parameters when encoding and decoding the same field. For more information, see base64 encoding options to decide which parameters to use.

base64Decode function

Performs Base64 decoding of the input string. The input is assumed to be a URL-safe Base64-encoded string.

Example - decode blob metadata or URLs

Your source data might contain Base64-encoded strings, such as blob metadata strings or web URLs, that you want to make searchable as plain text. You can use the base64Decode function to turn the encoded data back into regular strings when populating your search index.

"fieldMappings" : [
    "sourceFieldName" : "Base64EncodedMetadata",
    "targetFieldName" : "SearchableMetadata",
    "mappingFunction" : { 
      "name" : "base64Decode", 
      "parameters" : { "useHttpServerUtilityUrlTokenDecode" : false }

If you don't include a parameters property, it defaults to the value {"useHttpServerUtilityUrlTokenEncode" : true}.

Azure Search supports two different Base64 encodings. You should use the same parameters when encoding and decoding the same field. For more details, see base64 encoding options to decide which parameters to use.

base64 encoding options

Azure Search supports two different Base64 encodings: HttpServerUtility URL token, and URL-safe Base64 encoding without padding. A string that is base64-encoded during indexing should later be decoded with the same encoding options, or else the result won't match the original.

If the useHttpServerUtilityUrlTokenEncode or useHttpServerUtilityUrlTokenDecode parameters for encoding and decoding respectively are set to true, then base64Encode behaves like HttpServerUtility.UrlTokenEncode and base64Decode behaves like HttpServerUtility.UrlTokenDecode.

If you are not using the full .NET Framework (that is, you are using .NET Core or another framework) to produce the key values to emulate Azure Search behavior, then you should set useHttpServerUtilityUrlTokenEncode and useHttpServerUtilityUrlTokenDecode to false. Depending on the library you use, the base64 encoding and decoding functions might differ from the ones used by Azure Search.

The following table compares different base64 encodings of the string 00>00?00. To determine the required additional processing (if any) for your base64 functions, apply your library encode function on the string 00>00?00 and compare the output with the expected output MDA-MDA_MDA.

Encoding Base64 encode output Additional processing after library encoding Additional processing before library decoding
Base64 with padding MDA+MDA/MDA= Use URL-safe characters and remove padding Use standard base64 characters and add padding
Base64 without padding MDA+MDA/MDA Use URL-safe characters Use standard base64 characters
URL-safe base64 with padding MDA-MDA_MDA= Remove padding Add padding
URL-safe base64 without padding MDA-MDA_MDA None None

extractTokenAtPosition function

Splits a string field using the specified delimiter, and picks the token at the specified position in the resulting split.

This function uses the following parameters:

  • delimiter: a string to use as the separator when splitting the input string.
  • position: an integer zero-based position of the token to pick after the input string is split.

For example, if the input is Jane Doe, the delimiter is " "(space) and the position is 0, the result is Jane; if the position is 1, the result is Doe. If the position refers to a token that doesn't exist, an error is returned.

Example - extract a name

Your data source contains a PersonName field, and you want to index it as two separate FirstName and LastName fields. You can use this function to split the input using the space character as the delimiter.

"fieldMappings" : [
    "sourceFieldName" : "PersonName",
    "targetFieldName" : "FirstName",
    "mappingFunction" : { "name" : "extractTokenAtPosition", "parameters" : { "delimiter" : " ", "position" : 0 } }
    "sourceFieldName" : "PersonName",
    "targetFieldName" : "LastName",
    "mappingFunction" : { "name" : "extractTokenAtPosition", "parameters" : { "delimiter" : " ", "position" : 1 } }

jsonArrayToStringCollection function

Transforms a string formatted as a JSON array of strings into a string array that can be used to populate a Collection(Edm.String) field in the index.

For example, if the input string is ["red", "white", "blue"], then the target field of type Collection(Edm.String) will be populated with the three values red, white, and blue. For input values that cannot be parsed as JSON string arrays, an error is returned.

Example - populate collection from relational data

Azure SQL Database doesn't have a built-in data type that naturally maps to Collection(Edm.String) fields in Azure Search. To populate string collection fields, you can pre-process your source data as a JSON string array and then use the jsonArrayToStringCollection mapping function.

"fieldMappings" : [
    "sourceFieldName" : "tags", 
    "mappingFunction" : { "name" : "jsonArrayToStringCollection" }

For a detailed example that transforms relational data into index collection fields, see Model relational data.