Databases Reference
In-Depth Information
Thanks to the power of LINQ to Objects, the
UnleashedTableProvider
constructor packs
quite a bit of logic in just a few lines of code. Breaking it down into steps…the first part,
shown next, uses the static
TypeDescriptor
class to obtain a list of
PropertyDescriptor
objects that represent custom properties of the entity class:
IEnumerable<PropertyDescriptor> customProperties = TypeDescriptor
.GetProperties(original.EntityType).Cast<PropertyDescriptor>()
.Where(p => !p.PropertyType.IsSubclassOf(typeof(EntityReference))
.Where(p => this.RootEntityType.IsAssignableFrom(p.ComponentType))
.Where(p => original.Columns.All(c => c.Name != p.Name));
Several
Where
calls are used to filter out inapplicable properties. The first
Where
call
eliminates the “reference” properties that Entity Framework generates for every foreign
key property, such as the
SupplierReference
property, which accompanies the
Supplier
property. Whereas the
Supplier
property returns an object of type Supplier, the
SupplierReference
property returns an
EntityReference<Supplier>
that for the purpose
of filtering represents the same logical metadata column. The
second
Where
call removes
the properties declared in the
EntityObject
base class or higher in the inheritance hierar-
chy of the entity class, such as the
EntityKey
property. The last
Where
call removes all
entity properties already recognized by the original table provider:
this.columns = original.Columns
.Concat(customProperties.Select(p => new UnleashedColumnProvider(this, p)))
.ToList().AsReadOnly();
Having obtained
PropertyDescriptor
objects, the second part of the
UnleashedTableProvider
constructor creates an
UnleashedColumnProvider
object for each custom property and
“concatenates” them with the collection of
ColumnProvider
objects returned by the origi-
nal
TableProvider
. Stored in the private
columns
field, this combined collection is
returned by the
Columns
property overridden in this class.
To recap, the
UnleashedTableProvider
class implements the Decorator pattern and
extends the list of columns returned by the original table provider to include the
UnleashedColumnProvider
objects for custom properties defined in the entity class. All
other properties and methods overridden in the
UnleashedTableProvider
call the original
implementation, reusing and preserving the built-in logic.
Extending the DataModelProvider to Support Custom Properties
The
UnleashedModelProvider
inherits from the built-in
DataModelProvider
and encapsu-
lates metadata information describing one or more entity classes. Each
DataModelProvider
is responsible for creating and returning a collection of
TableProvider
objects. Just like the
MetaModel
class discussed earlier in this chapter, you must subclass the
DataModelProvider
whenever you subclass the
TableProvider
.
The
UnleashedModelProvider
also implements the Decorator pattern by wrapping an orig-
inal
DataModelProvider
instance. The main purpose of this class is to change the
Tables
