Mixins.md

####Mixins

Any reference properties, including nested and enums, with rules.maxOccurrences = 1, can have an optional boolean attribute - mixin. When true, all properties and custom methods of referenced class are semantically considered as belonging to the host object.

This technique allows to compose new classes from one or many source classes, making possible:

• emulation of single inheritance
• emulation of multiple inheritance

flexish_cli utility recognizes cases with possible mixin references when generating Flexilite schema from existing SQLite database. General rule is:

• there is a primary key on a single column
• there is a foreign key relation from the primary key column to another table.

For example, Nortwind sample database has tables Orders and InternationalOrders. Both tables have OrderID column, and in InternationalOrders it is also a foreign key to Orders.
This is considered by flexish_cli as sufficient ground to treat InternationalOrders as a sort of a superclass of Orders.

We will use these tables to demonstrate how mixins are organized in Flexilite.

In Flexilite, InternationalOrders is declared as class which has, among other, scalar properties, a mixin property called Order, with reference to Orders class. Complete list of properties of InternationalOrders is as follows:

• OrderID - enum property, and also a user-defined ID
• CustomsDescription - text
• ExciseTax - money
• OrderID_ref - auto generated internal reference property, paired with OrderID and pointing to Orders. It has mixin=true and this makes all properties of referenced object in Orders class (like CustomerID, EmployeeID, OrderDate etc.) to be directly available as properties of InternationalOrders.

Few more notes on mixins:

• rules.maxOccurrences must be 1. Other values are not allowed.
• Properties of mixin class can be accessed directly (without property name qualifier) as well as using property qualifier. For example, in triggers, queries or functions OrderDate can be accessed as InternationalOrders.OrderDate or InternationalOrders.Order.OrderDate. These constructs are identical.
• referenced objects with mixin=true cannot reference themselves directly or indirectly, i.e. recursion is not allowed.
• mixin objects are stored and manipulated as standalone objects belonging to their own class. This, in particular, means that they are indexed the same way as normal objects of the same class and participate in searches.

#####Host object Object that has mixin references is called host object. It can be referenced as mixin by other objects, thus ending in multi-level structure similar to class hierarchy. Top level object (the real host) can be referenced from any mixin objects as self, with access to all properties and methods.

Any given object may belong to single host object. Attempt to host an object which is already hosted, in another object, with raise an error. Multiple mixins are OK, as far as every single object is a mixin for only one host object.