# Structure of a generator assignment
In the Tonic Structural API, a generator assignment is referred to as a replacement.
A group of replacements makes up the message body for the response to get generator configuration details, and a request to update generator configuration details.
For details and examples of replacements for each Structural generator, go to [generator-api-reference](https://docs.tonic.ai/app/api/quick-start-guide/tonic-api-generator-assignment/generator-api-reference "mention").
## Replacement structure
At a very high level, the structure of a replacement object is:
Diagram that shows the overall outline of the replacement structure
Each [`Replacement`](https://app.tonic.ai/apidocs/index.html#/models/Replacement) object contains:
* The name of the replacement.
* The schema and table where the configured columns are located.
* Link objects for generator and sub-generator configurations.
* Columns to use for partitioning.
## Link object structure
Within a replacement, each [link](https://app.tonic.ai/apidocs/index.html#/models/Link) object contains the generator or sub-generator configuration for a single column.
For `fallBackLinks`, the link object contains the generator configuration for the fallback generator.
{% code overflow="wrap" %}
```json
{
"schema": "string",
"table": "string",
"column": "string",
"path": "string", //JSON fields only
"dataType": "string", //MongoDB, Amazon DynamoDB, and JSON fields only
"metadata": {
"presetId": "string",
"generatorId": "string",
"isConsistent": boolean,
"consistencyColumn": "string",
"isDifferentiallyPrivate": boolean,
//Other generator configuration fields
"encryptionProcessor": "x-on", //To use configured Structural data encryption
"customValueProcessor": "string", //If custom value processor applied
"pathExpression": "string", //Path expression for a composite generator
"subGeneratorMetadata": {
"presetId": "string",
"generatorId": "string",
"customValueProcessor": "string" //If custom value processor applied to the sub-generator
//Other generator configuration fields for the sub-generator
}
}
}
```
{% endcode %}
## Column identification
In the link object, to identify the column, you always provide the schema name, table name, and column name.
The `schema` and `table` values in the link object must match the `schema` and `table` values for the replacement.
Note that even if there isn't a schema (for example, for the Databricks data connector), you must still provide an empty value for `schema`.
### Identifying MongoDB and Amazon DynamoDB fields
For MongoDB and Amazon DynamoDB fields, in addition to the schema, table, and column, you use the `dataType` field to provide the data type.
### Identifying a field in a JSON column
For a JSON column, in the Structural application, you can [use **Document View** to assign a generator to individual JSON fields](https://docs.tonic.ai/app/generation/working-with-document-based-data/json-document-view).
To use the API to assign a generator to a JSON field, in addition to the schema, table, and column:
* You use the `path` field to provide the path to the field.
* You use the `dataType` field to provide the field data type.
## Metadata
In the link object, the `metadata` object identifies the generator and generator preset, and provides the generator configuration.
### Generator and preset identification
In the `metadata` object, `presetId` identifies the applied [generator preset](https://docs.tonic.ai/app/generation/generators-assign-config/generator-presets) configuration. `generatorId` identifies the type of generator. `generatorId` must match the generator type for `presetId`.
Generator presets require an Enterprise license. For Basic and Professional licenses, only `generatorId` is provided.
For the built-in preset for a generator, `presetId` and `generatorId` are the same. If during configuration the generator preset specified by `presetId` is not available - for example, if the generator preset was deleted - then the baseline version of the generator specified by `generatorId` is applied.
### Generator configuration
For the generator configuration, `metadata` contains fields from [`BaseMetadata`](https://app.tonic.ai/apidocs/index.html#/models/BaseMetadata), which provides the fields to configure consistency and differential privacy.
`metadata` can also contain additional objects and fields from generator-specific metadata objects.
### Structural data encryption
In the `metadata` object, if [Structural data encryption](https://docs.tonic.ai/app/generation/generators-assign-config/generators-data-encryption-config) is enabled for the instance, then to indicate to use the configured data encryption, set `encryptionProcessor` to `x-on`.
### Custom value processors
In the `metadata` object, you can specify a [custom value processor](https://docs.tonic.ai/app/generation/generators-assign-config/custom-value-processors) to apply to the generator (`customValueProcessor`).
### Sub-generator configuration
In the `metadata` object, for composite generators other than Array Regex, Regex, or Conditional, `pathExpression` identifies the value within the column to apply a sub-generator to.
The `subGeneratorMetadata` object then identifies and configures the generator to apply to that value:
```json
"pathExpression": "string",
"subGeneratorMetadata": {
"presetId": "string",
"generatorId": "string",
"customValueProcessor": "string"
//Other generator configuration fields for the sub-generator
}
```
Within `subGeneratorMetadata`:
* `presetId` identifies the generator preset to apply.
* `generatorId` identifies the type of generator.
* `customValueProcessor` identifies the custom value processor to apply.
`subGeneratorMetadata` also contains any other fields used to configure the selected sub-generator.
## Partition column list
The Continuous and Event Timestamps generators allow [partitioning](https://docs.tonic.ai/app/generation/generators/generator-characteristics/partitioning).
In the replacement object, the `partitions` field contains a comma-separated list of columns to partition by.
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