# Cross Table Sum (CrossTableAggregateGenerator)

The [Cross Table Sum](https://docs.tonic.ai/app/generation/generators/generator-reference/cross-table-sum) generator sets the value of the column to the sum of the values of another column aggregated across rows that have a foreign key value that matches the primary key in the current record.

For example, in a `users` table, a `total_transactions` value is obtained from the `transactions` table by combining all of the `transaction_amount` values from rows that have a `user_id` value that matches the primary key value for the current `users` record.

## Link object structure <a href="#generator-api-cross-table-sum-link-object" id="generator-api-cross-table-sum-link-object"></a>

The Cross Table Sum generator does not support linking or consistency. You cannot configure differential privacy.

The `metadata` object is populated from the [`CrossTableAggregateMetadata`](https://app.tonic.ai/apidocs/index.html#/models/CrossTableAggregateMetadata) object.

The generator-specific configuration includes:

* The schema and table that contain the column to sum against.
* The foreign key column to compare against the primary key for the current table.
* The column that contains the values to sum.
* The primary key column in the current table.

```json
{
  "schema": "string",
  "table": "string",
  "column": "string",
  "metadata": {
    "generatorId": "CrossTableAggregateGenerator",
    "foreignSchema": "string",
    "foreignTable": "string",
    "foreignKeyColumns": [ "string" ],
    "sumColumn": "string",
    "primaryKeyColumns": [ "string" ],
    "encryptionProcessor": "x-on", //To use configured Structural data encryption
    "customValueProcessor": "string" //If custom value processor applied 
  }
}
```

## Example replacement <a href="#generator-api-cross-table-sum-replacement" id="generator-api-cross-table-sum-replacement"></a>

In the following example replacement for the Cross Table Sum generator, the value of `total_transactions` in the `users` table is set to the sum of the values of the `amount` column in the `transactions` table for rows where `user_id` has the same value as the `id` column in the current `users` table row.

```json
{
  "name": "total-transactions",
  "schema": "public",
  "table": "users",
  "links": [
    {
      "schema": "public",
      "table": "users",
      "column": "total_transactions",
      "metadata": {
        "generatorId": "CrossTableAggregateGenerator",
        "foreignSchema": "public",
        "foreignTable": "transactions",
        "foreignKeyColumns": [ "user_id" ],
        "sumColumn": "amount",
        "primaryKeyColumns": [ "id" ]
      }
    }
  ]
}
```