Updated the Field Mappings article. Added .NET SDK section. Referenced other example articles. Edited and fixed some typos.

RobDixon22 · RobDixon22 · commit 34dd7b0529b2 · 2019-06-13T14:05:21.000-07:00
diff --git a/articles/search/search-indexer-field-mappings.md b/articles/search/search-indexer-field-mappings.md
@@ -14,26 +14,37 @@ ms.custom: seodec2018
 ---
 
 # 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.
+
+When using Azure Search indexers, you sometimes find that your 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 `_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.   
+* 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.
+
+> [!NOTE]
+> 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 transform 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](search-example-adventureworks-modeling.md) and [Model multilevel facets](search-example-adventureworks-multilevel-faceting.md).
+>
 
-## Setting up field mappings
-You can add field mappings when creating a new indexer using the [Create Indexer](https://msdn.microsoft.com/library/azure/dn946899.aspx) API. You can manage field mappings on an indexing indexer using the [Update Indexer](https://msdn.microsoft.com/library/azure/dn946892.aspx) API.
+## 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](#mappingFunctions).
 
-Fields mappings are added to the `fieldMappings` array on the indexer definition.
+Field mappings are added to the `fieldMappings` array of the indexer definition.
 
-For example, here's how you can accommodate differences in field names:
+## Map fields using the REST API
+
+You can add field mappings when creating a new indexer using the [Create Indexer](https://msdn.microsoft.com/library/azure/dn946899.aspx) API request. You can manage the field mappings of an existing indexer using the [Update Indexer](https://msdn.microsoft.com/library/azure/dn946892.aspx) API request.
+
+For example, here's how to map a source field to a target field with a different name:
 
 ```JSON
 
@@ -47,7 +58,7 @@ api-key: [admin key]
 }
 ```
 
-An indexer can have multiple field mappings. For example, here's how you can "fork" a field:
+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:
 
 ```JSON
 
@@ -62,10 +73,37 @@ An indexer can have multiple field mappings. For example, here's how you can "fo
 >
 >
 
+## Map fields using the .NET SDK
+
+You define field mappings in the .NET SDK using the [FieldMapping](https://docs.microsoft.com/dotnet/api/microsoft.azure.search.models.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.
+
+```C#
+  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);
+```
+
 <a name="mappingFunctions"></a>
 
 ## Field mapping functions
-These functions are currently supported:
+
+A field mapping function transforms the contents of a field before it is stored in the index. The following mapping functions are currently supported:
 
 * [base64Encode](#base64EncodeFunction)
 * [base64Decode](#base64DecodeFunction)
@@ -74,13 +112,14 @@ These functions are currently supported:
 
 <a name="base64EncodeFunction"></a>
 
-## base64Encode
+### base64Encode function
+
 Performs *URL-safe* Base64 encoding of the input string. Assumes that the input is UTF-8 encoded.
 
-### Sample use case - 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](https://docs.microsoft.com/rest/api/searchservice/lookup-document), 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. Once the key is encoded, you can use base64 decode to retrieve the original value. For details, see the [base64 encoding and decoding](#base64details) section.
+#### 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](https://docs.microsoft.com/rest/api/searchservice/lookup-document) ). 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. Once the key is encoded, you can use base64 decode to retrieve the original value. For details, see the [base64 encoding and decoding](#base64details) section.
 
-#### Example
 ```JSON
 
 "fieldMappings" : [
@@ -91,51 +130,59 @@ Only URL-safe characters can appear in an Azure Search document key (because cus
   }]
 ```
 
-### Sample use case - retrieve original key
+#### Example - retrieve original key
+
 You have a blob indexer that indexes blobs with the blob path metadata as the document key. After retrieving the encoded document key, you want to decode the path and download the blob.
 
-#### Example
 ```JSON
 
 "fieldMappings" : [
   {
     "sourceFieldName" : "SourceKey",
     "targetFieldName" : "IndexKey",
-    "mappingFunction" : { "name" : "base64Encode", "parameters" : { "useHttpServerUtilityUrlTokenEncode" : false } }
+    "mappingFunction" : {
+      "name" : "base64Encode",
+      "parameters" : { "useHttpServerUtilityUrlTokenEncode" : false }
+    }
   }]
  ```
 
 If you don't need to look up documents by keys and also don't need to decode the encoded content, you can just leave out `parameters` for the mapping function, which defaults `useHttpServerUtilityUrlTokenEncode` to `true`. Otherwise, see [base64 details](#base64details) section to decide which settings to use.
 
 <a name="base64DecodeFunction"></a>
 
-## base64Decode
-Performs Base64 decoding of the input string. The input is assumed to a *URL-safe* Base64-encoded string.
+### 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
 
-### Sample use case
-Blob custom metadata values must be ASCII-encoded. You can use Base64 encoding to represent arbitrary UTF-8 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.
+Blob custom metadata values must be ASCII-encoded. You can use Base64 encoding to represent arbitrary UTF-8 strings in blob custom metadata. To make search data more meaningful, you can use the base64Decode function to turn the encoded data back into regular strings when populating your search index.
 
-#### Example
 ```JSON
 
 "fieldMappings" : [
   {
     "sourceFieldName" : "Base64EncodedMetadata",
     "targetFieldName" : "SearchableMetadata",
-    "mappingFunction" : { "name" : "base64Decode", "parameters" : { "useHttpServerUtilityUrlTokenDecode" : false } }
+    "mappingFunction" : { 
+      "name" : "base64Decode", 
+      "parameters" : { "useHttpServerUtilityUrlTokenDecode" : false }
+    }
   }]
 ```
 
-If you don't specify any `parameters`, then the default value of `useHttpServerUtilityUrlTokenDecode` is `true`. See [base64 details](#base64details) section to decide which settings to use.
+If you don't specify any `parameters`, then the default value of `useHttpServerUtilityUrlTokenDecode` is `true`. See the [base64 encoding options](#base64details) section to decide which settings to use.
 
 <a name="base64details"></a>
 
-### Details of base64 encoding and decoding
-Azure Search supports two base64 encodings: HttpServerUtility URL token and URL-safe base64 encoding without padding. You need to use the same encoding as the mapping functions if you want to encode a document key for look up, encode a value to be decoded by the indexer, or decode a field encoded by the indexer.
+### base64 encoding options
 
-If `useHttpServerUtilityUrlTokenEncode` or `useHttpServerUtilityUrlTokenDecode` parameters for encoding and decoding respectively are set to `true`, then `base64Encode` behaves like [HttpServerUtility.UrlTokenEncode](https://msdn.microsoft.com/library/system.web.httpserverutility.urltokenencode.aspx) and `base64Decode` behaves like [HttpServerUtility.UrlTokenDecode](https://msdn.microsoft.com/library/system.web.httpserverutility.urltokendecode.aspx).
+Azure Search supports two different Base64 encodings: **HttpServerUtility URL token**, and **URL-safe Base64 encoding without padding**. You must use the same encoding as the mapping functions to encode a document key for lookup, encode a value that will be decoded by the indexer, or decode a field that was encoded by the indexer.
 
-If you are not using the full .NET Framework (i.e., you are using .NET Core or other programming environment) 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 encode and decode utility functions may be  different from Azure Search.
+If the `useHttpServerUtilityUrlTokenEncode` or `useHttpServerUtilityUrlTokenDecode` parameters for encoding and decoding respectively are set to `true`, then `base64Encode` behaves like [HttpServerUtility.UrlTokenEncode](https://msdn.microsoft.com/library/system.web.httpserverutility.urltokenencode.aspx) and `base64Decode` behaves like [HttpServerUtility.UrlTokenDecode](https://msdn.microsoft.com/library/system.web.httpserverutility.urltokendecode.aspx).
+
+If you are not using the full .NET Framework (that is, you are using .NET Core or other programming environment) 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 encode and decode utility 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`.
 
@@ -148,19 +195,21 @@ The following table compares different base64 encodings of the string `00>00?00`
 
 <a name="extractTokenAtPositionFunction"></a>
 
-## extractTokenAtPosition
+### 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.
 
-### 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.
+#### Example - extract a name
 
-### 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.    
+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.
 
-### Example
 ```JSON
 
 "fieldMappings" : [
@@ -178,22 +227,23 @@ Your data source contains a `PersonName` field, and you want to index it as two
 
 <a name="jsonArrayToStringCollectionFunction"></a>
 
-## jsonArrayToStringCollection
+### 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.
 
-### 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.
+#### 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.
 
-### Example
 ```JSON
 
 "fieldMappings" : [
-  { "sourceFieldName" : "tags", "mappingFunction" : { "name" : "jsonArrayToStringCollection" } }
-]
+  {
+    "sourceFieldName" : "tags", 
+    "mappingFunction" : { "name" : "jsonArrayToStringCollection" }
+  }]
 ```
 
-
-## Help us make Azure Search better
-If you have feature requests or ideas for improvements, please reach out to us on our [UserVoice site](https://feedback.azure.com/forums/263029-azure-search/).
+For a detailed example that transforms relational data into index collection fields, see [Model relational data](search-example-adventureworks-modeling.md).
