LLBLGen Pro - generated code - Entity collection and Typed List/Typed View paging, Adapter

Generated code - Entity collection and Typed List/Typed View paging, Adapter

Preface

Paging is the way to browse through a list of objects or rows of data one page at a time. This can be handy when you have thousands of rows / objects matching search criteria but you want to enlist only a small number at once. With the paging functionality build into the DataAccessAdapter class, you can tell the generated code which page to retrieve for typed lists, typed views or entity collections, instead of getting all the results at once. This section describes the various options you have.

Note:

On SqlServer 7 and 2000, paging is implemented using temp tables. This is done to keep one codebase for both SqlServer 7 and SqlServer 2000 and it gives reasonable performance in all situations (small/large resultsets). Paging using ROWCOUNT tricks is not possible due to the fact that this kind of paging is pretty limited when it comes to compound primary keys. On SqlServer 2005, paging is done through a CTE query. Please refer to Generated code - Application configuration through .config files and Generated code - Database specific features how to set the SqlServer DQE into SqlServer 2005 compatibility mode so it will use a CTE based query instead of a temp table based query.

Paging through an entity collection

Paging through an entity collection is implemented in an overload of DataAccessAdapter.FetchEntityCollection(). The particular overload accepts the page size, which is the number of objects to retrieve by the fetch action, and the page number to retrieve. If you for example pass 10 for the page size and 4 for the page number, you'll get record number 31-40, the first record is 1, the first page is also numbered 1. Paging is disabled if you pass 0 for the page number or 0 or 1 for the page size.

Get the total number of objects

To effectively use paging, it is key to know how many pages there are. For example, you want to show a list of page numbers the user can choose from, like Google uses. You can retrieve the number of objects matching your filter by using the DataAccessAdapter's GetDbCount() method. The method below uses an Aggregate function. Read more about aggregate functions and expressions in the section Field expressions and aggregates. The example retrieves the number of different order objects of customers from France.

C#
VB.NET, .NET 1.x
VB.NET, .NET 2.0

// C#
IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.PredicateExpression.Add(CustomerFields.Country == "France");
filter.Relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId);
DataAccessAdapter adapter = new DataAccessAdapter();
int amount = (int)adapter.GetDbCount(new OrderEntityFactory().CreateFields(), filter, null, false);

' VB.NET .NET 1.x
Dim filter As IRelationPredicateBucket = New RelationPredicateBucket()
filter.PredicateExpression.Add(New FieldCompareValuePredicate( _
	CustomerFields.Country, Nothing, ComparisonOperator.Equal, "France"))
filter.Relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId)
Dim adapter As New DataAccessAdapter()
Dim amount As Integer = CInt(adapter.GetDbCount(new OrderEntityFactory().CreateFields(), filter, Nothing, False))

' VB.NET .NET 2.0
Dim filter As IRelationPredicateBucket = New RelationPredicateBucket()
filter.PredicateExpression.Add(CustomerFields.Country = "France")
filter.Relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId)
Dim adapter As New DataAccessAdapter()
Dim amount As Integer = CInt(adapter.GetDbCount(new OrderEntityFactory().CreateFields(), filter, Nothing, False))

The value in amount can now be used to calculate the total number of pages when the page size is given: total number of pages = (total number of objects / pagesize) + n, where n is either 0 (total number of objects modulo pagesize is 0) or 1 (total number of objects modulo pagesize > 0). Below is the code to retrieve page 4, with a pagesize of 10 objects. We re-use the filter objects used in the GetScalar() call:

C#
VB.NET

// C#
EntityCollection orders = new EntityCollection(new OrderEntityFactory());
adapter.FetchEntityCollection(orders, filter, 0, null, 4, 10);

' VB.NET
Dim orders As New EntityCollection(New OrderEntityFactory())
adapter.FetchEntityCollection(orders, filter, 0, Nothing, 4, 10)

After this call, orders contains 10 objects, which formed the 4th page in the result set matching the filter defined. No sorting is applied here, but if you specify a sort expression, the sorting is performed prior to the paging logic.

Paging through a TypedList or TypedView

The paging functionality is also available for typed list and typed view classes, through an overload of the FetchTypedList() method for typed lists and an overload of FetchTypedView for typed views. For typed lists and typed views, the same definitions are valid as for collections: page numbers start at 1, the first record is numbered 1 and paging is disabled if you pass in a page number of 0 or you pass in a page size of 0 or 1.

Get the total number of rows

Getting the total number of rows for a typed list, is a bit different than it is for a collection. Instead of creating a new RelationPredicateBucket object, you use the RelationPredicateBucket returned from the typed list's method GetRelationInfo(). To that, you add your predicates. For typed views, it works the same as enlisted above for entities. Below is the code to get the number of rows in a typed list with order-customer rows.

C#
VB.NET

// C#
OrderCustomerTypedList orderCustomer = new OrderCustomerTypedList();
IEntityFields2 fields = orderCustomer.GetFieldsInfo();
IRelationPredicateBucket filter = orderCustomer.GetRelationInfo();
filter.PredicateExpression.Add(CustomerFields.Country == "France");
DataAccessAdapter adapter = new DataAccessAdapter();
int amount = (int)adapter.GetDbCount(fields, filter, null, false);

' VB.NET
Dim orderCustomer As New OrderCustomerTypedList()
Dim fields As IEntityFields2 = orderCustomer.GetFieldsInfo()
Dim filter As IRelationPredicateBucket = orderCustomer.GetRelationInfo()
filter.PredicateExpression.Add(New FieldCompareValuePredicate( _
	CustomerFields.Country, Nothing, ComparisonOperator.Equal, "France"))
Dim adapter As New DataAccessAdapter()
Dim amount As Integer = CInt(adapter.GetDbCount(fields, filter, Nothing, False))

Fetching a given page in the typed list or typed view is then boiling down to using the FetchTypedList() overload which accepts the two paging parameters for fetching a typed list' page or calling the proper FetchTypedView method for fetching the required page for a typed view. Be sure to clear the typed list/typed view object before calling the fetch methods again to fetch another page.