model the domain objects    

If you are going to create an object to represent a real life entity inside your OBEROn application then you must define first which are the characteristics for this object and actions that can be done with it. In other words, each object is an instance of a well-defined object model and it is described by an object class.
A class identifies a kind of application object and the collection of properties that characterize it. Like for an object programming language, a class is a model composed of attributes (or fields) that collect the object data and methods that operate on that data.

Object classes are basically defined by the fields they contain. Objects may have fields such as dimensions, volume, weight, color, materials, shape and so on.
You can define a field as a characteristic of a class of object or of a link between two objects. For example, assume the object is a movie. It might have attributes such as author, director, producer, duration time, date of publication, etc.
When an object is created, the user specifies the object class and so he can introduce the field values for that object instance. The specific value for each field can be different for each object instance; however, all objects of the same class have the same fields.

A class can be derived from another class or in other terms it extends this class. This means that the extended class is of the same kind as its parent (called SuperClass). For example, a thriller is a kind of movie which in turn is a type of media. The structure of derived classes is called a class hierarchy. Extended classes inherit characteristics from their parent classes and in particular they have at minimum the same fields and methods in addition to the own ones. This concept is called field/method inheritance.

Best practice is to create first the basic classes and then derive the extended ones. A class can be also defined Abstract or non-Abstract: abstract classes are like subset of properties for other extended classes. They are useful only in defining characteristics that are inherited by other object classes, but cannot be used to create any object instances. At the contrary, non-abstract classes can be used to create instances of business objects.

Users enabled to manage Classes (see user access rights ) can create a new class instance by clicking the "Add new Class" button from the "Context Design" menu and compile the relative form.

The same users should also have access to create Field instances to add to the new class.

When you define a field, the Type option is always required: it identifies the type of values the attribute will have. A field can assume seven different types of values:

Once a type is assigned to a field, it can be changed, but in some cases this operation causes a loss of information.
You can also define a Default value for the field. When a business object is created and you don't set the field value, the default value is assigned. When assigning a default value, the value you give must agree with the attribute type. You you set the default = today for date and datetime field types, the value will be set automatically to the object/link creation date(time).

Real values are usually expressed on some kind of Unit Measure; when a unit measure for a real field is selected, OBEROn will use the standard international (SI) metric system to represent the value inside the database. Alternative unit measures can be associated to a real field, but their usage depends on the user default metric system. In other terms, if a field allows an alternative UM and this UM is included into the user's metric system, the field value will be converted from the SI unit to the user unit during the read operation and converted back from the user unit to the corresponding SI unit during the save/write operation.

Also the date and datetime types can have an associated unit measure, but in this case it represents the date format.

The Reset option represents the behaviour of the field value when the object instance is cloned or revised; there are the following settings:

In addition you can define the Range of values the field can assume. When a field has a specific range, any value the user tries to assign to that field is checked to determine if it is within that range. Only values that respect the defined range are allowed. The default value must also be valid for the defined ranges.

There are two types of range values: some are or inclusive operators while other are constraints. A value is valid if respects all the defined constraints and at least one of the defined conditions (or possible values) in the range list.

When a field has the "Multiple Values" option active its value can be the concatenation of more values separated by the comma character (",") and each value must be valid respect to the range list as seen above.

You can create a field with an OOQL command like the following:

To complete the Movie class creation, you can search the fields just created into the "Fields" tab of the class form and add them.

The equivalent OOQL command is:

As the "Movie" class is abstract you cannot instantiate objects of this kind and you have to define at least one extended class derived from it. For example, you can introduce a classification by genre and define a sub-class for each type of movie: Comedy, Romance, Horror, Thriller, Drama and so on.

For a particular type of movie you might need more fields to accurately describe the content. For example, over the years horror movies have been divided in different categories: one classification finds them as supernatural, scientific and naturalistic movies. The field HorrorType is added to the inherited ("H") fields from the basic Movie class.