Designer - Inheritance mapping

Preface

LLBLGen Pro offers full inheritance mapping starting with version 1.0.2005.1. To be able to map inheritance hierarchies in an easy, productive way, the designer offers various ways to create inheritance hierarchies, which are discussed in the following paragraphs. It's important you've read the section Concepts - Entity inheritance and relational models so you're familiar with the names used in this section. Not only the creation of hierarchies is discussed, also the destruction of (parts of) the hierarchies is described.

Creating hierarchies of type TargetPerEntity

To create a hierarchy of type TargetPerEntity, you have two options. One is the option to let LLBLGen Pro find all hierarchies of TargetPerEntity in the entities in the project. To start this option, right-click Entities in the project explorer and select Construct 'Target-per-entity' Hierarchies as shown in the following screenshot.



Creating all TargetPerEntity hierarchies in one go, before

After you've selected the option, LLBLGen Pro will find all hierarchies of type TargetPerEntity and will construct them for you. On the example database in the screenshot, one hierarchy was found, Employee <- Manager <- BoardMember, Employee <- Clerk, which gives the following project explorer overview



Creating all TargetPerEntity hierarchies in one go, after

Which is the hierarchy shown in the screenshot for the Inheritance sub tab in the section Adding and editing entities. The automatical construction of entity hierarchies rejects entities which are in a typed list so the user won't run into the situation where the hierarchy has to be destroyed which isn't allowed for entities which are in a typedlist. A warning is shown which entities were rejected and why. A warning is also shown when an entity is made a subtype of another entity manually and one or both are in a typed list.

Another option to create a hierarchy of type TargetPerEntity is to build the hierarchy per entity. You activate this option by right-clicking an entity and then selecting the context menu option Make Sub-type of which will show you an entity type which is a candidate to be a supertype for the currently selected entity. See the screenshot below for an example.



Making an entity a subtype of another entity in a hierarchy of type TargetPerEntity

After selecting the suggested supertype from the context menu, LLBLGen Pro will create the hierarchy for you and make the selected entity in the project explorer a subtype of the entity selected from the context menu.

Creating hierarchies of type TargetPerEntityHierarchy

Hierarchies of type TargetPerEntityHierarchy are all mapped, per hierarchy, onto the same target. This means that creating a hierarchy of type TargetPerEntityHierarchy is done differently than the creation of hierarchies of type TargetPerEntity, as described in the previous paragraph. To create a hierarchy of type TargetPerEntityHierarchy, you right-click in the project explorer the entity which will become the supertype of a newly created subtype, and then you select the option Create Sub-type For This Entity from the context menu, as shown in the following screenshot



Creating a subtype of another entity in a hierarchy of type TargetPerEntityHierarchy

Selecting that option will bring up the following dialog which will allow you to specify the discriminator values and entity name for the subtype. If the entity you right-clicked in the project explorer isn't in a hierarchy of type TargetPerEntityHierarchy, you also have to specify the discriminator value for that entity.



Dialog for creating a subtype of another entity in a hierarchy of type TargetPerEntityHierarchy

The screenshot above shows that the discriminator column is CarType, which is of type System.Int32. The selected discriminator column's type is leading for which discriminator values are allowed. LLBLGen Pro will list all columns which have as .NET type byte/int16/int32/int64/Guid/Decimal and string. All known types in the hierarchy are also shown in the dialog to help you specify unique values for the discriminator values. In the example above, the hierarchy is new, and no types are defined yet.

By clicking OK, the subtype will be created as a new entity, mapped onto the same target as the supertype, and the new entity is defined as a subtype of the chosen supertype.

Note:

Discriminator fields shouldn't be foreign key fields. The reason for this is that by setting a different related entity, the type of the entity could change because the discriminator field value changes, which is impossible as that would mean the entity object in memory should also change type dynamically.

LLBLGen Pro will automatically unmap in the supertype all fields which are nullable, if the supertype is the root of the hierarchy. This is to help you setup the hierarchy in less time, as in TargetPerEntityHierarchy hierarchies, it's typical that fields mapped in subtypes are nullable in the target. No fields are mapped in the newly created subtype, you've to specify them yourself in the entity editor. See Designer - Adding and editing entities.

Besides right-clicking the supertype entity in the project explorer, you can also right-click an entity in the hierarchy view on an entity's Inheritance info tab, in the entity editor. See for more details Designer - Adding and editing entities.

Viewing hierarchies

After a hierarchy is created, you can view the hierarchy in the project explorer by clicking open the entity nodes of entities in the hierarchy and then by expanding the Sub-types nodes as shown in the following screenshot.



Hierarchy in project explorer

Another way of viewing the hierarchy is to open the entity editor of one of the entities in the hierarchy, which will show the complete hierarchy in the Inheritance info tab of the entity editor, as shown in this section: Designer - Adding and editing entities

Destroying hierarchies

Creating hierarchies also means that from time to time, hierarchies have to be destroyed or parts of hierarchies have to be removed. Destroying a hierarchy isn't the same as deleting an entity from the project: by deleting an entity from the project, it's automatically removed from the hierarchies it's in, and the subtypes of the entity to delete are automatically removed from the hierarchy as well, however removing an entity from a hierarchy simply means that the entity isn't in the hierarchy after the action has been completed, it will become a separate entity in the project.

An entity which isn't a leaf in the hierarchy (i.e.: it has at least one subtype), can't be removed from the hierarchy alone: all its subtypes have to be removed as well. To do so, you right-click in the project explorer the entity from which you want the hierarchy to be destroyed and select Destroy Hierarchy, Starting With This Entity from the context menu, as shown in the following screenshot.



Destroying a hierarchy

After selecting this option, all entities below the selected entity and the selected entity will be removed from the hierarchy they're in and will become separate entities in the project. You can't undo this option. This option is also selectable from the context menu of the hierarchy viewer on the Inheritance info tab in the entity editor.

If you right-click an entity which is a leaf, e.g. it doesn't have any subtypes, you can remove it from the hierarchy its in by simply selecting Remove From Hierarchy, which will make it a separate entity in the project. You can't undo this option. If the leaf entity was in a hierarchy of type TargetPerEntityHierarchy you can't re-add it to the hierarchy after the action.