Using Conditional Mapping in Entity Framework

June 15, 2010

2 comments

Using Conditional Mapping in Entity Framework


There are timesUsing Conditional Mapping in Entity Framework
that we want to
retrieve only
a portion of data
from a table in
the database
permanently by some
filter. For that purpose
we can use the conditional mapping in Entity Framework.


What is Conditional Mapping?


Conditional mapping is a fixed condition that helps use to
filter the result set that is being returned from the database
for a specific entity. Also it enforces that an entity is mapped
to data in the database under only certain conditions which are
supplied inside the conditional mapping.
In order to use conditional mapping we need to open the
Mapping Details View. In the view we can add for each entity
a conditional mapping using the <Add a Condition>:
Conditional Mapping
When we create a condition, that condition will be added to each
query that we will make to the database.


Available Conditional Mappings


There are some possible conditional mapping in Entity Framework
which are provided by two kind of operators – the equality operator
and the Is operator.
The equality operator (=) can have values of strings or integers.
The Is operator check whether a column is Null or Not Null.
When we have more then one condition it will construct an And
operation between all the conditions.


Conditional Mapping Example


A very common example for using a conditional mapping is having
a table field that indicate a logical delete (for example an IsDeleted
field). Since we want to present only undeleted rows/entities
then a conditional mapping can be a valid solution.

In the example I’m using the following table that represent a course data:
Course Table
The IsDeleted has a default value of 0 to indicate that a row’s logical
state is not deleted. If the row is deleted I change the value to 1.

This is the entity in the generated model before I make any
changes:
Course Entity
Now I want to impose the conditional mapping on the course entity.
The first thing to do is to delete the IsDeleted property since it
will be used by the conditional mapping. Then I create a conditional
mapping
that retrieve only entities with IsDeleted equals to 0 in
the following way:
The Conditional Mapping

That is it.
Now whenever I query for courses, the generated query will be
created with a where clause that check whether IsDeleted equals 0.
For example if I query for all courses the following query will be sent to
the database:



SELECT 
[Extent1].[CourseID] AS [CourseID], 
[Extent1].[Title] AS [Title], 
[Extent1].[Days] AS [Days], 
[Extent1].[Time] AS [Time], 
[Extent1].[Location] AS [Location], 
[Extent1].[Credits] AS [Credits], 
[Extent1].[DepartmentID] AS [DepartmentID]
FROM [dbo].[Course] AS [Extent1]
WHERE [Extent1].[IsDeleted] =  CAST( ’0′ AS tinyint)

Since I use tinyint as data type in the database then there is a
casting in the query.


Summary


A conditional mapping in Entity Framework helps to filter returning
results. If you have a fixed behavior such as logical delete then
conditional mapping can help you achieve the relevant behavior.

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2 comments

  1. thomekJuly 7, 2010 ב 10:18

    A very helpfull post, thanks.
    I was wondering how you could approach the actual logical deletion in this scenario: the IsDeleted property will no longer be available in the entity. So how do you logically delelete a record in this case? Via a trigger in the database?

    Thanks,
    thomek

    Reply
  2. Gil FinkJuly 7, 2010 ב 18:24

    @thomek,

    You can provide a stored procedure in order to do the logical delete from Entity Framework. No trigger is needed.

    Reply