Closed Enumerations
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A closed enumeration should be used when a fixed set of values must be defined as part of the schema. With a closed enumeration, an application that takes a dependency on the schema (version) can take a dependency on these values.
Design Pattern
Closed enumerations can be defined using a type as part of the schema, or as an extent populated with an initial set of values, which is then secured to prevent update – access control being used to “close” what is otherwise an open enumeration.
An enumeration can be defined in-line as an anonymous type, or as a reusable type which is then referenced explicitly. A reusable type, with a singular name, should be defined if the same enumeration with the same meaning is referenced more than once. In either case, the enumeration is expressed as a collection of values. Most simply, the collection can be used directly as a type, in which case the data type is inferred from the values. Alternatively an explicit data type can be referenced and the collection used in a membership constraint.
When using an enumeration to define a stored field, consideration should be given to the space required. Meaningful text values are appropriate if readability of the stored values is more important than space efficiency. If space is a concern (particularly if the enumeration values are long text strings or there are high volumes involved) a short codified value or alias scheme should be considered. Using small integer alias values results in the most compact storage; however, if alias values are used, they will require translation on update and query.
In the following code, the field ContactKind is declared as an enumeration in-line with meaningful text values. The field Gender is declared inline using an explicit type and a membership constraint over a series of short text codes. PrimaryPhoneUse and SecondaryPhoneUse use an integer-based enumeration defined by the type PhoneUse where the data type is inferred from the values.
module Patterns.Enumerations.Closed
{
People :
{
Id : Integer64 = AutoNumber();
Name : Text where value.Count <= 100;
ContactKind :
{
"Customer",
"Colleague",
"Friend"
};
Gender : {"M", "F"};
PrimaryPhone : (Text where value.Count <= 20)?;
PrimaryPhoneUse : PhoneUse?;
SecondaryPhone : (Text where value.Count <= 20)?;
SecondaryPhoneUse : PhoneUse?;
}* where identity Id;
type PhoneUse
{
// Home
1,
// Work
2,
// Unknown
3
};
}