Field mappings in Azure Search indexers

When using Azure Search indexers, you can occasionally find yourself in situations where your input data doesn't quite match the schema of your target index. In those cases, you can use field mappings to transform your data into the desired shape.

Some situations where field mappings are useful:

  • Your data source has a field _id, but Azure Search doesn't allow field names starting with an underscore. A field mapping allows you to "rename" a field.
  • You want to populate several fields in the index with the same data source data, for example because you want to apply different analyzers to those fields. Field mappings let you "fork" a data source field.
  • You need to Base64 encode or decode your data. Field mappings support several mapping functions, including functions for Base64 encoding and decoding.

Setting up field mappings

You can add field mappings when creating a new indexer using the Create Indexer API. You can manage field mappings on an indexing indexer using the Update Indexer API.

A field mapping consists of 3 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.

Fields mappings are added to the fieldMappings array on the indexer definition.

For example, here's how you can accommodate differences in field names:

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

An indexer can have multiple field mappings. For example, here's how you can "fork" a field:

"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.

Field mapping functions

These functions are currently supported:


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

Sample use case

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, for example). If your data contains URL-unsafe characters and you want to use it to populate a key field in your search index, use this function.


"fieldMappings" : [
    "sourceFieldName" : "Path",
    "targetFieldName" : "UrlSafePath",
    "mappingFunction" : { "name" : "base64Encode" }


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

Sample use case

Blob custom metadata values must be ASCII-encoded. You can use Base64 encoding to represent arbitrary Unicode strings in blob custom metadata. However, to make search meaningful, you can use this function to turn the encoded data back into "regular" strings when populating your search index.


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


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

Sample use case

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.


  • 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.


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


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 will be returned.

Sample use case

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, format your source data as a JSON string array and use this function.


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

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