Generated code - Entity collection and Typed List/Typed View paging, SelfServicing
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 entity collection classes and typed list/typed view
classes, you can tell the generated code which page to retrieve, instead of getting all the results at once. This section describes the
various options you have.
 Note:
|
|
On SqlServer 2000, paging is implemented using temp tables. This is done
as 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 GetMulti(). The particular overload accepts the page size, which is the number of objects to
retrieve in 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 entity collection method GetDbCount().
the method has overloads to support filtering spanning multiple entities as well. Below is an example to retrieve the total number of different order objects of
customers from France.
// C#
OrderCollection orders = new OrderCollection();
IPredicateExpression filter = new PredicateExpression();
filter.Add(CustomerFields.Country == "France");
IRelationCollection relations = new RelationCollection();
relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId);
int amount = orders.GetDbCount(filter, relations);
' VB.NET
Dim orders As New OrderCollection()
Dim filter As IPredicateExpression = New PredicateExpression()
filter.Add(CustomerFields.Country = "France")
Dim relations As IRelationCollection = New RelationCollection()
relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId)
Dim amount As Integer = orders.GetDbCount(filter, relations)
The number is retrieved by using the GetScalar() method of the entity collection class, using the Count(*) aggregate function. See for more details about
using Aggregate functions in your code the section
Field expressions and aggregates.
The value in
amount can now be used to calculate the total number of pages when the page size is given: number of pages = (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 GetDbCount() call:
// C#
orders.GetMulti(filter, 0, null, relations, 4, 10);
' VB.NET
orders.GetMulti(filter, 0, Nothing, relations, 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 Fill() method. 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 or typed view works the same as it is done for a collection. A typed list and typed view have
a method called GetDbCount and various overloads accept a filter, and for typed lists als a RelationCollection.
You can also specify if the GetDbCount should take into account duplicate rows or not.
Fetching a given page in the typed list or typed view is then boiling down to using the Fill() overload which accepts the two paging parameters. Be sure to
clear the typed list/typed view object before calling Fill() again to fetch another page.
