> ## Documentation Index
> Fetch the complete documentation index at: https://flatfileinc-feat-updatereactquickstart.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Constraints

> System level validation rules

## Field-Level Constraints

Field-level constraints allow you to indicate additional validations that will be applied to fields of a Sheet. These constraints are in addition to the implicit constraints based on types of data. (eg: String, Number)

For example, a value is required for every record (RequiredConstraint), or every record must contain a unique value for the field (UniqueConstraint).

### Required Constraint

Required Constraints indicate that the given field **must** be provided with a non-null value.

<Note>A `null` value in this case constitutes an empty cell.</Note>

```json
// within the context of a create Workbook API call
 "fields": [
  {
    "key": "firstName",
    "type": "string",
    "label": "First Name",
    "constraints": [
      {
        "type": "required"
      }
    ]
  }
]
```

### Unique Constraint

Unique Constraints indicate that the given field value must only appear **once** in all the values for that field.

<Note>
  `null` values will appear many times because they are not considered unique as
  there is no value to compare.
</Note>

```json
// within the context of a create Workbook API call
 "fields": [
  {
    "key": "internalCustomerId",
    "type": "number",
    "label": "Internal Customer Id",
    "constraints": [
      {
        "type": "unique"
      }
    ]
  }
]
```

### Computed Value

You might notice a ComputedValue type within the Blueprint code. Please do not use this value at this time as it is unsupported and may be deprecated without notice.

## Sheet-Level

### Composite Uniqueness

Composite Uniqueness is a feature where a combination of two or more fields must be unique across an entire Sheet.

To implement composite uniqueness, add the following parameters to the `constraints` property:

* `name` *defines the name of the constraint*

* `fields` *array of field keys to consider*

* `type` *defines the type of the constraint*

* `strategy` *defines how to determine uniqueness*
  {" "}
  Can either be:
  {" "}
  `concat` *concatenates values from each field defined inside the `fields` array*
  {" "}
  or `hash` *creates a sha1 hash using of the values of each field*

```json
"sheets": [
  {
    "name": "Products",
    "description": "A list of products available for sale",
    "slug": "products",
    "constraints": [
      {
        "name": "constraint name",
        "fields": ["field_key_one", "field_key_two"],
        "type": "unique",
        "strategy": "concat"
      }
    ],
  }
]
```