Validating User Input

We start with the annotation @mandatory. Elements marked with @mandatory are checked for nonempty input: null and (trimmed) empty strings are rejected.

In the example shown, the title element of the Books entity is annotated with @mandatory. If you try to create a book without a title, the corresponding request returns a response with HTTP status code 400 (Bad Request). The response contains an error message similar to the following:

123456
"error": {
  "code": "400",
  "message": "Value is required",
  "target": "title",
  "@Common.numericSeverity": 4
}

The @assert.target annotation can be used for managed to-one associations of a CDS model entity. The annotation is employed to check during CREATE and UPDATE operations whether the target entity to which the association refers exists. In other words, use this annotation to ensure that a non-null foreign key in a table has a corresponding primary key in the associated/referenced target table.

In the example shown, the managed to-one association author of the Books entity is annotated with @mandatory and @assert.target. The @mandatory annotation ensures that a book author is specified. The @assert.target annotation also checks whether the specified author exists.

If you try to create a new book and the author ID specified in the request does not exist in the database, the requested CREATE operation will not be executed. Instead, the HTTP response has the status code 400 (Bad Request), as we have already seen with the @mandatory annotation. The response body contains an error message similar to the following. It points to the invalid foreign key:

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"error": {
  "code": "400",
  "message": "Value doesn't exist",
  "target": "author_ID",
  "@Common.numericSeverity": 4
}

The @assert.range annotation allows the specification of value ranges for elements of model entities with ordinal types - i.e. numeric or date/time types. The value range is defined as an interval with the following syntax:

1
@assert.range: [min, max]

The defined range is regarded as a closed interval, i.e. it is checked whether a specified value is greater than or equal to min and less than or equal to max.

For enum elements, true can be specified to restrict possible values to the defined enum values.

In the example shown, the genre element of the Books entity has the data type Genre, which is a custom type with enumeration values. Annotating the genre element with @assert.range: true ensures that only values from the enumeration can be specified for this element, i.e. 1 or 2.

If, for example, the value 3 is provided for the genre element when creating a book, the requested CREATE operation is not executed. Instead, the HTTP response has the status code 400 (Bad Request) and the response body contains an error message similar to the following:

123456
"error": {
  "code": "400",
  "message": "Value 3 is invalid according to enum declaration {1, 2}",
  "target": "genre",
  "@Common.numericSeverity": 4
}

Watch the video to see how to implement input validation.