JSON mapping

Applies to: ✅ Microsoft Fabric ✅ Azure Data Explorer

Use JSON mapping to map incoming data to columns inside tables when your ingestion source file is in JSON format.

Each element in the mapping list defines the mapping for a specific column. These elements are constructed from three properties: column, datatype, and properties. Learn more in the data mappings overview.

Each JSON mapping element must contain either of the following optional properties:

Property	Type	Description
Path	string	If the value starts with $ it's interpreted as the JSON path to the field in the JSON document that will become the content of the column in the table. The JSON path that denotes the entire document is $. If the value doesn't start with $ it's interpreted as a constant value. JSON paths that include special characters should be escaped as ['Property Name']. For more information, see JSONPath syntax.
ConstValue	string	The constant value to be used for a column instead of some value inside the JSON file.
Transform	string	Transformation that should be applied on the content with mapping transformations.

Important

For queued ingestion:

• If the table referenced in the mapping doesn't exist in the database, it gets created automatically, given that valid data types are specified for all columns.
• If a column referenced in the mapping doesn't exist in the table, it gets added automatically to the table as the last column upon the first time data is ingested for that column, given a valid data type is specified for the column. To add new columns to a mapping, use the .alter ingestion mapping command.
• Data is batched using Ingestion properties. The more distinct ingestion mapping properties used, such as different ConstValue values, the more fragmented the ingestion becomes, which can lead to performance degradation.

Examples

[
  {"Column": "event_timestamp", "Properties": {"Path": "$.Timestamp"}},
  {"Column": "event_name",      "Properties": {"Path": "$.Event.Name"}},
  {"Column": "event_type",      "Properties": {"Path": "$.Event.Type"}},
  {"Column": "source_uri",      "Properties": {"Transform": "SourceLocation"}},
  {"Column": "source_line",     "Properties": {"Transform": "SourceLineNumber"}},
  {"Column": "event_time",      "Properties": {"Path": "$.Timestamp", "Transform": "DateTimeFromUnixMilliseconds"}},
  {"Column": "ingestion_time",  "Properties": {"ConstValue": "2021-01-01T10:32:00"}},
  {"Column": "full_record",     "Properties": {"Path": "$"}}
]

The mapping above is serialized as a JSON string when it's provided as part of the .ingest management command.

.ingest into Table123 (@"source1", @"source2")
  with
  (
      format = "json",
      ingestionMapping =
      ```
      [
        {"Column": "column_a", "Properties": {"Path": "$.Obj.Property"}},
        {"Column": "column_b", "Properties": {"Path": "$.Property"}},
        {"Column": "custom_column", "Properties": {"Path": "$.[\'Property name with space\']"}}
      ]
      ```
  )

Pre-created mapping

When the mapping is pre-created, reference the mapping by name in the .ingest management command.

.ingest into Table123 (@"source1", @"source2")
    with
    (
        format="json",
        ingestionMappingReference = "Mapping_Name"
    )

Identity mapping

Use JSON mapping during ingestion without defining a mapping schema (see identity mapping).

.ingest into Table123 (@"source1", @"source2")
    with
    (
        format="json"
    )

Copying JSON mapping

You can copy JSON mapping of an existing table and create a new table with the same mapping using the following process:

1. Run the following command on the table whose mapping you want to copy:

   .show table TABLENAME ingestion json mappings
   | extend formatted_mapping = strcat("'",replace_string(Mapping, "'", "\\'"),"'")
   | project formatted_mapping
   

2. Use the output of the above command to create a new table with the same mapping:

   .create table TABLENAME ingestion json mapping "TABLENAME_Mapping" RESULT_OF_ABOVE_CMD