There are times 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>:



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:



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:



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:



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.