Generated code - Using the EntityView class, SelfServicing

Preface

The EntityView is a class which is used to create in-memory views on an entity collection object and allows you to filter and sort an in-memory entity collection without actually touching the data inside the entity collection. A collection can have multiple EntityView objects, similar to the DataTable - DataView combination. This section describes how to use the EntityView class in various different scenarios.

DataBinding and EntityViews

Entity collections don't bind directly to a bound control. They always bind through their EntityView object (returned by the property DefaultView, see below). This is a change from the approach taken by LLBLGen Pro 1.0.2005.1 and earlier, where an entity collection was always bound directly to a bound control. The EntityView approach allows you to create multiple EntityViews on a single entity collection and all bind them to different controls as if they're different sets of data.

Creating an EntityView instance

Creating an EntityView object is simple


This creates an EntityView object on the entity collection customers, so it lets you view the data in the entity collection 'customers'. EntityView objects don't contain any data: all data you'll be able to access through an EntityView is actually data residing in the related entity collection.

You can also use the entity collection's DefaultView property to create an EntityView. This is similar to the DataTable's DefaultView property: every time you read the property, you'll get the same view object back. This is also true for the entity collection's DefaultView property:


The DefaultView property calls the virtual method CreateDefaultEntityView() which you can override to customize the EntityView instance created for 'DefaultView'.

Instead of using the EntityView class, you can use the IEntityView interface, if you for example don't know the generic type in .NET 2.0 code.
The EntityView constructor has various overloads which let you specify an initial filter and / or sort expression. You can also set the filter and / or sort expression later on as described below. Please familiar yourself with the various methods and properties of the EntityView class, by checking its entry in the LLBLGen Pro reference manual.

Filtering and sorting an EntityView

The purpose of an EntityView is to give you a 'view' based on a filter and / or a sortexpression on an in-memory entity collection. Which data contained in the related entity collection is available to you through a particular EntityView object depends on the filter set for the EntityView. In which order the data is available to you is controlled by the set sort expression. As the related collection is not touched, you can have as many EntityView objects on the same entity collection, all exposing different subsets of the data in the entity collection, in different order.

Filtering and sorting an EntityView is done through normal LLBLGen Pro predicate and sortclause classes. See for more information about predicate classes: Getting started with filtering and The predicate system.

The following example filters the aforementioned customers collection on all customers from the UK:


You also could have specified this filter with the EntityView constructor. As soon as the EntityView's property Filter is set to a value, the EntityView object resets itself and will apply the set IPredicate to the related entity collection and all matching entity objects will be available through the EntityView object.

This is similar to the EntityView's sorter. Let's sort our filtered EntityView on CompanyName, ascending. For more information about sortclauses and sortexpression objects, please see: Generated code - Sorting.

Use a Predicate(Of T) or Lambda expression (.NET 3.5+) for a filter

In .NET 2.0, Microsoft introduced a new class called Predicate<T>. This is a class which is used in a couple of methods in List<T> and Array for example. In .NET 3.5, Lambda expressions were introduced, which are actually Func<T, U> (and variants) implementations. The .NET 3.5+ compilers will compile a lambda expression to a Predicate<T> if the method requires a Predicate<T>, as both are under the surface simply delegates. EntityView2 has a couple of constructors which accept a Predicate<T>. This allows you to specify a lambda expression in .NET 3.5+ to filter the entity collection, or if you're on .NET 2.0/3.0, you can use a delegate which compiles to Predicate<T>. The example below filters the passed in collection of CustomerEntity instances on the Country property:


Using the DelegatePredicate<T>, a developer can also use a Predicate<T> delegate or Lambda expression to filter the EntityView2 instance after it's been created:

Multi-clause sorting

Entity collection classes by themselves offer a Sort() method, which is there for backwards compatibility and was used in previous versions by the IBindingList.ApplySort() method. The Sort() method however has one drawback: it can only sort on a single field or property. What if you want to sort on multiple fields? As the EntityView allows you to sort the data using a SortExpression, you can specify as much fields as you want. Let's sort the customerView on City ascending and on CompanyName descending:


What if you want to sort on a property of an entity, which isn't an entity field? After all, Sort() allows you to do that. This is also possible: to specify a property, you've to use the class EntityProperty instead of an entity field. So if you instead of sorting on CompanyName, want to sort on the entity property IsDirty, to get all the changed entities first, and then the non-changed entities, you've to use this code instead:


EntityProperty is usable in any construct which works with an entityfield, as long as it's in-memory sorting or filtering. Below you'll learn how to filter an EntityView's data using an entity property.

Filtering using multiple predicates

As a PredicateExpression derives from Predicate, you can also use a PredicateExpression to filter using multiple predicates. There's a limitation however: not all predicate classes are usable for in-memory filtering: please consult the section Generated code - The predicate system which classes are usable and with which specifics. The filtering is also focussed on the entities inside the related entity collection, not on entities inside those entities. This thus means you can't specify a RelationCollection for example to filter all Customers who have an Order from last May.

To filter the customers collection on all customers from the UK which entities have been changed, use the following code:

View behavior on collection changes

When an entity changes in the related entity collection of the EntityView, it can be the entity doesn't match anymore with the filter set for the view and the EntityView therefore removes the entity from itself: it's no longer available to you through the EntityView. This can be confusing so it is definable what the EntityView should do when the data inside the related entity collection changes. This is done by specifying a PostCollectionChangeAction value with the EntityView constructor or by setting the EntityView's DataChangeAction property. The following list describes the various values and their result on the EntityView's behavior:

• NoAction (do nothing), i.e.: don't re-apply the filter nor the sorter.
• ReapplyFilterAndSorter (default). Reapplies the filter and sorter on the collection.
• ReapplySorter. Reapplies the sorter on the collection, not the filter.

By default, the EntityView will re-apply the filter and sorter. There's no setting for just the filter, as re-applying the filter could alter the set, which could change the order of the data as in: it's no longer ordered and has to be re-sorted. If the related collection fires a reset event (when it is sorted using its own code or cleared), the view is also reset and filters are re-applied as well as sorters.

If a new entity is added to the collection through code, it is not added to the view in NoAction mode or in ReapplySorter mode, because no filter is re-applyed. If it's added through databinding, it actually is added to the view, as it is added through the EntityView, because an entity collection is bound to a bound control via an EntityView, either an EntityView object you created and bound directly or through the EntityView object returned by the entity collection's DefaultView property.

Projecting data inside an EntityView on another data-structure

A powerful feature of the EntityView class is the ability to project the data in the EntityView onto a new data-structure, like an entity collection, datatable or even custom classes. Projections are a way to produce custom lists of data ('dynamic lists in memory') based on the current data in the EntityView and a collection of projection objects. Projection objects are small objects which specify which entity field or entity property should be used in the projection and where to get the value from. For example, because the raw projection data can be used to re-instantiate new entities, the data can be used to produce a new entity collection with new entities. How the data is projected depends on the projection engine used for the actual projection. For more information about projections please also see: LLBLGen Pro - Fetching DataReaders and projections.

Projections are performed by applying a set of projection objects onto an entity and then by passing on the result data array for further storage to a projection engine, or projector, the projected data is placed in a new instance of a class, for example an entity class, but this can also be a DataRow or a custom class. (Projections on custom classes are only supported on .NET 2.0). The array is an array of type object. You can use filters during the projection as well, to limit the set of data you want to project from the EntityView data. In .NET 1.x, you've to use ArrayList objects to provide the projector objects. In .NET 2.0, you can use the generic List(Of T) class.

Projection objects: EntityPropertyProjector

A projection object is an instance of the EntityPropertyProjector class. As EntityView objects contain Entity objects, this is the projection object you should use. LLBLGen Pro supports other projection objects as well, for general purpose projections as discussed in Fetching DataReaders and projections, however these aren't usable with EntityViews. An EntityPropertyProjector instance contains at most two IEntityFieldCore instances (for example normal EntityField objects or an EntityProperty object) and a Predicate, for example a FieldCompareValuePredicate, or a PredicateExpression. The first IEntityFieldCore instance is mandatory. This is the default value. If a Predicate is specified (optional), and it resolves to true, the default value (thus the first IEntityFieldCore) is used, otherwise the second IEntityFieldCore instance.

This way you can select per entity from two fields, for example SomeEntity.Name1 and SomeEntity.Name2, based on the predicate specified either the value of field Name1 of the entity, if the predicate resolves to true, otherwise the value of Name2.

The EntityPropertyProjector also contains a Name property which is used to produce the name of the result field. The projection routine used is free to use this name for column purpose (projection onto a datatable) but can also use it for entity field setting (projection onto an entity).

If a developer wants to execute a piece of code onto the value prior to storing it into the projected slot, the developer can derive his own class from EntityPropertyProjector and override ValuePostProcess(). This routine is normally empty and expects the value and the entity being processed. It all might sound a little complex, but it's fairly straigt forward, as will be shown in a couple of examples below.

Projecting an EntityView's data is done by the CreateProjection routine of an EntityView object. LLBLGen Pro comes with three different projection engines: one for projecting data onto a DataTable (the class DataProjectorToDataTable), one for projecting data onto an entity collection (the class DataProjectorToEntityCollection) and on .NET 2.0 also one for projecting data onto a list of custom classes (the class DataProjectorToCustomClass). You can write your own projection engine as well: simply implement the interface IEntityDataProjector to be able to use the engine in projections of EntityView data. If you also want to use the same engine in projections of resultsets as discussed in Fetching DataReaders and projections, you also should implement the almost similar interface IGeneralDataProjector. Because the interfaces can re-use the actual projection engine logic, it's easy to re-use projection code for both projection mechanisms.

Only the data which is available to you through the EntityView can possibly be projected. You can't project nested data inside entities nor entity data not in the EntityView. In that case, create a new EntityView on the same entity collection using a different filter and project that EntityView object instead.

Creating EntityPropertyProjector instances for all entity fields.
Sometimes you want to project all fields of a given entity and it can be cumbersome to create a lot of EntityPropertyProjector objects if your entity has a lot of fields. Instead, you can use the shortcut method on EntityFields2: EntityFields2.ConvertToProjectors( EntityFieldsFactory.CreateEntityFieldsObject(EntityType.entitynameEntity))

This method will return List of IEntityPropertyProjector objects, one for each entity field of the specified entity type.

Examples of EntityView projections

Projection to datatable.


After this code, the datatable projectionResults contains two columns, City and CustomerID, and it contains the data for the fields City and CustomerId of each entity in the EntityView, which are all entities with Country equal to "Germany".

Projection to entity collection
The following example performs a projection onto an entity collection. Clerk is another subtype of Employee.


After this code, the collection clerks contains ClerkEntity instances with only the EmployeeEntity fields (inherited by ClerkEntity from its base type EmployeeEntity, which is also the base type of ManagerEntity) filled with data.

Projection to custom classes
With some reflection, it is possible to create such a class for .NET 1.x, though the class itself has to be setup a little different. The code below also shows how to use the projectors in .NET 2.0. It uses the class TestCustomer which is given below the projection example code (in C#). The projection also shows how to project a property of an entity which isn't an entity field, namely IsDirty, using the EntityProperty class.


The custom class, TestCustomer:

 
/// 

/// Test class for projection of fetched entities onto custom classes using a custom projector.
/// 
public class TestCustomer
{
	#region Class Member Declarations
	private string _customerID, _companyName, _city, _country;
	private bool _isDirty;
	#endregion

	public TestCustomer()
	{
		_city = string.Empty;
		_companyName = string.Empty;
		_customerID = string.Empty;
		_country = string.Empty;
		_isDirty = false;
	}

	#region Class Property Declarations
	public string CustomerID
	{
		get { return _customerID; }
		set { _customerID = value; }
	}

	public string City
	{
		get { return _city; }
		set { _city = value; }
	}

	public string CompanyName
	{
		get { return _companyName; }
		set { _companyName = value; }
	}

	public string Country
	{
		get { return _country; }
		set { _country = value; }
	}

	public bool IsDirty
	{
		get { return _isDirty; }
		set { _isDirty = value; }
	}

	#endregion	
}

Distinct projections.

It can be helpful to have distinct projections: no duplicate data in the projection results. Distinct projections are supported, as the following example will show. Creating a distinct projection is simply passing false / False for allowDuplicates in the CreateProjection method.

The following example shows a couple of projection related aspects: it filters the entity view's data using a Like predicate prior to projecting data, so you can limit the data inside an EntityView used for the projection, and it shows an example how a predicate is used to choose between two values in an entity to determine the end result of projecting an entity. The example uses Northwind like most examples in this documentation. The code contains Assert statements, which are left to show you how many elements to expect at that point in the routine.

CustomerCollection customers = new CustomerCollection();
customers.GetMulti(null);
EntityView customersInGermanyView = 
	new EntityView( customers, (CustomerFields.Country == "Germany"), null );
Assert.AreEqual( 11, customersInGermanyView.Count );

// create straight forward projection of these customers of just the city and the customerid.
ArrayList propertyProjectors= new ArrayList();
propertyProjectors.Add( new EntityPropertyProjector( CustomerFields.City, "City" ) );
propertyProjectors.Add( new EntityPropertyProjector( CustomerFields.CustomerId, "CustomerID" ) );
DataTable projection = new DataTable();
customersInGermanyView.CreateProjection( propertyProjectors, projection );
Assert.AreEqual( 11, projection.Rows.Count );

// do distinct filtering during the following projection. It projects ContactTitle and IsNew
propertyProjectors = new ArrayList();
propertyProjectors.Add( new EntityPropertyProjector( CustomerFields.ContactTitle, "Contact title" ) );
// any entity property can be used for projection source.
propertyProjectors.Add( new EntityPropertyProjector( new EntityProperty( "IsNew" ), "Is new" ) );
projection = new DataTable();
customersInGermanyView.CreateProjection( propertyProjectors, projection, false );
Assert.AreEqual( 7, projection.Rows.Count );

// do distinct filtering and filter the set to project. Re-use previous property projectors. 
// 3 rows match the specified filter, distinct filtering makes it 2.
projection = new DataTable();
customersInGermanyView.CreateProjection( propertyProjectors, 
	projection, false, (CustomerFields.ContactTitle % "Marketing%") );
Assert.AreEqual( 2, projection.Rows.Count );

// use alternative projection source based on filter.
projection = new DataTable();
propertyProjectors = new ArrayList();
// bogus data, but performs what we need: for all contacttitles not matching the filter, CustomerId is used.
propertyProjectors.Add( new EntityPropertyProjector( CustomerFields.ContactTitle, 
		"Contact title", (CustomerFields.ContactTitle % "Marketing%"), CustomerFields.CustomerId) );
propertyProjectors.Add( new EntityPropertyProjector( CustomerFields.CustomerId, "CustomerID" ) );
// create a new projection, with distinct filtering, which gives different results 
// now, because ContactTitle is now sometimes equal to CustomerId
customersInGermanyView.CreateProjection( propertyProjectors, projection, false );
Assert.AreEqual( 11, projection.Rows.Count );
foreach( DataRow row in projection.Rows )
{
	if( !row["Contact title"].ToString().StartsWith( "Marketing" ) )
	{
		Assert.AreEqual( row["Contact title"], row["CustomerID"] );
	}
}

Aggregates aren't supported in in-memory projections though Expressions are. All expressions are fully evaluated, where '+' operators on strings result in string concatenations. The new DbFunctionCall object to call database functions inside an Expression object is ignored during expression evaluation.