The only validation keyword is used to restrict the value of a field to a specific set of allowed values.
The only keyword can be added to a field definition in the schema and accepts either a single allowed value, an array of allowed values, or a function returning an allowed value or array of allowed values.
{
fieldName: {dType: "String", only: "only valid value"}
}In the example above, the fieldName field must be equal to "only valid value".
When an array is provided, the field value must be one of the values in the array.
{
fieldName: {dType: "String", only: ["Tim", "Tom"]}
}In the above example, the fieldName field must be either "Tim" or "Tom".
If a function is provided, it will be called with the parameters (value, contract, dType, depth), and should return an allowed value or array of allowed values.
{
fieldName: {dType: "String", only: () => { /* your logic here */ }}
}The only validator considers a field valid if its value is exactly equal to the allowed value(s), or if the field is empty (undefined, null, or an empty string/array depending on the dType).
The error message for only is dynamic and indicates the allowed value(s). For a single allowed value, the message is "must be "{allowedValue}". For multiple allowed values, the message is "must be "{value1}", "{value2}", or "{lastValue}".
The i18next keys for these validation errors are errors:only.singular for a single allowed value, and errors:only.plural for multiple allowed values. For internationalization (i18n), you can provide translations for these keys in your i18n configuration.