5 Record Descriptors

Certain properties of a set of records can be specified by preceding them with a record descriptor. A record descriptor is itself a record, and uses fields with some predefined names to store the properties. The most basic property that can be specified for a set of records is their type. The special field name %rec is used for that purpose:

     %rec: Entry
     
     Id: 1
     Name: Entry 1
     
     Id: 2
     Name: Entry 2

The records following the descriptors are then identified as having its type. So in the example above we would say there are two records of type “Entry”.

The effect of a record descriptor ends when another descriptor is found in the stream of records. This allows to store different kind of records in the same database. For example, consider you have to maintain a depot. You will need to keep records of both the current stockage and the movements.

The following example shows the usage of two record descriptors to store both kind of records: articles and movements.

     %rec: Article
     
     Id: 1
     Title: Article 1
     
     Id: 2
     Title: Article 2
     
     %rec: Movement
     
     Id: 1
     Type: sell
     Date: 20 April 2011
     
     Id: 2
     Type: adquisition
     Date: 21 April 2011

Besides determining the type of the records that follows in the stream, record descriptors can be used to describe other properties of those records. That can be done by using the so-called special fields, having special names from a predefined set. Consider for example the following database, where the descriptor is used to specify a primary key and a mandatory field:

     %rec: Item
     %key: Id
     %mandatory: Title
     
     Id: 10
     Title: Notebook (big)
     
     Id: 11
     Title: Fountain Pen

Note that the names of special fields always start with the character %. Also note that it is also possible to use non-special fields in a record descriptor, but such fields will have no effect on the described record set.

What follows is an exhaustive list of the supported special fields. They are discussed in deep in the following sections.

%rec

Used to name the type of a set of records and to mark record descriptors.

%mandatory

%prohibit

Those special fields are used to control which fields can and cannot be included in records.

%unique

%key

Used to specify unique fields and primary keys.

%auto

Used to automatically generate certain fields when creating new records, like auto counter and time-stamps.

%doc

Used to describe the contents of a record set.

%typedef

%type

Used to define named types and to associated types with fields.

%sort

Used to keep your records sorted by some given field.

%size

Used to control the dimensions of a record set.

%confidential

Used to store confidential information.