# Fallback transform function
## Introduction
The `fallback` transform function is used to provide a value to be used if a source field to be mapped is empty. For example, consider the following payload:
```json
[
{"ref1": "10001","ref2":"10001"},
{"ref1":"20001", "ref2":"20001"},
{"ref1": "10001", "ref2":"10001"},
{"ref1": "30001", "ref2":"30001"},
{"ref1": "10001", "ref2":"10001"},
{"ref1": "", "ref2":"40001"}
]
```
Now, suppose that you are syncing the `ref1` field to a target system that cannot accept empty values. Given that the final item in this payload is empty, a process flow would fail.
One way to prevent this is to provide a fallback value - for example, `00000`. In this case, our incoming payload would be output as follows:
```json
[
{"ref1": "10001","ref2":"10001"},
{"ref1":"20001", "ref2":"20001"},
{"ref1": "10001", "ref2":"10001"},
{"ref1": "30001", "ref2":"30001"},
{"ref1": "10001", "ref2":"10001"},
{"ref1": "00000", "ref2":"40001"}
]
```
Using the `fallback` transform function, you can define fallback values using:
* [a static value](#adding-a-fallback-transform)
* [a flow variable value](#adding-a-fallback-transform)
* [a payload field value](#using-another-payload-field-for-fallback-values)
## Adding a fallback transform
{% stepper %}
{% step %}
**Edit map shape settings**
In your process flow, access settings for your map shape. For example:
{% endstep %}
{% step %}
**Access transforms for the required mapping**
Locate the mapping rule that you want to update (or add a new mapping rule), then click the `transform` button. For example:
{% endstep %}
{% step %}
**Add a new transform**
Click the `add transform` button. For example:
{% endstep %}
{% step %}
**Select the fallback transform function**
Click in the `name` field to access a list of all available transform functions, then select `fallback` from the `other` category. For example:
{% endstep %}
{% step %}
**Enter your required fallback value**
In the `fallback` field, enter the value to be used if the mapped source field is empty. You can enter a static value - for example:
Alternatively, you can pass in the value of a flow variable, using [flow variable syntax](https://doc.wearepatchworks.com/product-documentation/process-flows/building-process-flows/dynamic-variables/flow-variables) - for example:
You can also fall back to the value of another payload field however, this works slightly differently and you don't enter anything here. For details, please see [Using another payload field as a fallback value](#using-another-payload-field-as-a-fallback-value).
{% endstep %}
{% step %}
**Confirm the source field to be transformed**
Click the `add field` button:
...then click in `source fields` and select the source field to be used for this transform:
{% endstep %}
{% step %}
**Confirm transform settings**
Accept your changes (twice), then save the transform.
{% endstep %}
{% endstepper %}
## Using another payload field for fallback values
Consider the following payload:
```json
{"ref1": "", "ref2":"40001","ref3":"40001"}
```
Suppose we need to map `ref1` to a target field named `reference`. There's no value for `ref1` but we do have a value for `ref2`, which could be used as fallback. Now consider another payload for the same scenario:
```json
{"ref1": "", "ref2":"","ref3":"40001"}
```
There's no value for `ref1` or `ref2` - but we do have a value for `ref3`, which could be used as fallback.
You can specify one or more fields from the incoming payload(s) to be checked for a fallback value. To do this, follow the steps below.
{% stepper %}
{% step %}
**Edit map shape settings**
In your process flow, access settings for your map shape. For example:
{% endstep %}
{% step %}
**Define a mapping rule using all possible source fields**
Add a mapping rule in the usual way, with the required source and target fields. For example:
Then click anywhere in the source field:
...and choose to add another source field - for example:
Click anywhere in the new source field:
...and enter or select a field that can be checked for a fallback value - for example:
Accept the new field to exit back to the mapping rule - now you have two source fields mapped to the target:
If required, you can add additional 'fallback' source fields in the same way.
{% endstep %}
{% step %}
**Access transforms for the mapping rule**
Click the `transform` button for this mapping rule. For example:
{% endstep %}
{% step %}
**Add a new transform**
Click the `add transform` button. For example:
{% endstep %}
{% step %}
**Select the fallback transform function**
Click in the `name` field to access a list of all available transform functions, then select `fallback` from the `other` category. For example:
{% endstep %}
{% step %}
**Add the primary source field to be used**
Click the `add field` button:
...then click in `source fields` and select the source field to be used for this transform as a priority - for example:
{% hint style="info" %}
All source fields that you added in step 2 are available for selection.
{% endhint %}
Select the source field that should be checked for a value first, and accept this change. The field is added as the first to be transformed - you can now add the remaining source fields in the sequence you want them to be checked for a fallback value:
{% endstep %}
{% step %}
**Confirm transform settings**
Having added all source fields for this mapping rule, accept your changes - for example:
{% endstep %}
{% endstepper %}
