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Adding documentation for custom function mapping
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Fixes #500
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maumar committed Dec 10, 2020
1 parent 2667d74 commit 0f8f1c6
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41 changes: 41 additions & 0 deletions entity-framework/core/modeling/entity-types.md
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Expand Up @@ -116,3 +116,44 @@ You can set an arbitrary text comment that gets set on the database table, allow
[!code-csharp[Main](../../../samples/core/Modeling/FluentAPI/TableComment.cs?name=TableComment&highlight=4)]

***

## Table-valued function mapping

It's possible to map an entity type to a table-valued function (TVF) instead of a table in the database. To illustrate this, let's define another entity that represents blog with multiple posts. In the example, the entity is [keyless](xref:core/modeling/keyless-entity-types), but it doesn't have to be.

[!code-csharp[Main](../../../samples/core/Modeling/Conventions/EntityTypes.cs#BlogWithMultiplePostsEntity)]

Next, create the following table-valued function in the database, which returns only blogs with multiple posts as well as the number of posts associated with each of these blogs:

```sql
CREATE FUNCTION dbo.BlogsWithMultiplePosts()
RETURNS TABLE
AS
RETURN
(
SELECT b.Url, COUNT(p.BlogId) AS PostCount
FROM Blogs AS b
JOIN Posts AS p ON b.BlogId = p.BlogId
GROUP BY b.BlogId, b.Url
HAVING COUNT(p.BlogId) > 1
)
```

Now, the entity `BlogWithMultiplePost` can be mapped to this function in a following way:

[!code-csharp[Main](../../../samples/core/Modeling/Conventions/EntityTypes.cs#QueryableFunctionConfigurationToFunction)]

> [!NOTE]
> In order to map an entity to a table-valued function the function must be parameterless. Also, the names of the entity properties must match the names of the columns returned by the TVF; any discrepancies can be configured using `HasColumnName` method, just like when mapping to a regular table.
When the DbSet is mapped to a table-valued function, the query:

[!code-csharp[Main](../../../samples/core/Modeling/Conventions/Program.cs#ToFunctionQuery)]

Produces the following SQL:

```sql
SELECT [b].[Url], [b].[PostCount]
FROM [dbo].[BlogsWithMultiplePosts]() AS [b]
WHERE [b].[PostCount] > 3
```
140 changes: 140 additions & 0 deletions entity-framework/core/querying/user-defined-function-mapping.md
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---
title: User-defined function mapping - EF Core
description: Mapping user-defined functions to database functions
author: maumar
ms.date: 11/23/2020
uid: core/user-defined-function-mapping
---
# User-defined function mapping

EF Core allows for using user-defined SQL functions in queries. To do that, the functions need to be mapped to a CLR method during model configuration. When translating the LINQ query to SQL, the user-defined function is called instead of the CLR function it has been mapped to.

## Mapping a method to a SQL function

To illustrate how user-defined function mapping work, let's define the following entities:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#Entities)]

And the following model configuration:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#EntityConfiguration)]

Blog can have many posts and each post can have many comments.

Next, create the user-defined function `CommentedPostCountForBlog`, which returns the count of posts with at least one comment for a given blog, based on the blog `Id`:

```sql
CREATE FUNCTION dbo.CommentedPostCountForBlog(@id int)
RETURNS int
AS
BEGIN
RETURN (SELECT COUNT(*)
FROM [Posts] AS [p]
WHERE ([p].[BlogId] = @id) AND ((
SELECT COUNT(*)
FROM [Comments] AS [c]
WHERE [p].[PostId] = [c].[PostId]) > 0));
END
```

To use this function in EF Core, we define the following CLR method, which we map to the user-defined function:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#BasicFunctionDefinition)]

The body of the CLR method is not important. The method will not be invoked client-side, unless EF Core can't translate its arguments. If the arguments can be translated, EF Core only cares about the method signature.

> [!NOTE]
> In the example, the method is defined on `DbContext`, but it can also be defined as a static method inside other classes.
This function definition can now be associated with user-defined function in the model configuration:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#BasicFunctionConfiguration)]

By default, EF Core tries to map CLR function to a user-defined function with the same name. If the names differ, we can use `HasName` to provide the correct name for the user-defined function we want to map to.

Now, executing the following query:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Program.cs#BasicQuery)]

Will produce this SQL:

```sql
SELECT [b].[BlogId], [b].[Rating], [b].[Url]
FROM [Blogs] AS [b]
WHERE [dbo].[CommentedPostCountForBlog]([b].[BlogId]) > 1
```

## Mapping a method to a custom SQL

EF Core also allows for user-defined functions that get converted to a specific SQL. The SQL expression is provided using `HasTranslation` method during user-defined function configuration.

In the example below, we'll create a function that computes percentage difference between two integers.

The CLR method is as follows:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#HasTranslationFunctionDefinition)]

The function definition is as follows:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#HasTranslationFunctionConfiguration)]

Once we define the function, it can be used in the query. Instead of calling database function, EF Core will translate the method body directly into SQL based on the SQL expression tree constructed from the HasTranslation. The following LINQ query:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Program.cs#HasTranslationQuery)]

Produces the following SQL:

```sql
SELECT 100 * (ABS([p].[BlogId] - 3) / ((CAST([p].[BlogId] AS float) + 3) / 2))
FROM [Posts] AS [p]
```

## Mapping a queryable function to a table-valued function

EF Core also supports mapping to a table-valued function using a user-defined CLR method returning an `IQueryable` of entity types, allowing EF Core to map TVFs with parameters. The process is similar to mapping a scalar user-defined function to a SQL function: we need a TVF in the database, a CLR function that is used in the LINQ queries, and a mapping between the two.

As an example, we'll use a table-valued function that returns all posts having at least one comment that meets a given "Like" threshold:

```sql
CREATE FUNCTION dbo.PostsWithPopularComments(@likeThreshold int)
RETURNS TABLE
AS
RETURN
(
SELECT [p].[PostId], [p].[BlogId], [p].[Content], [p].[Rating], [p].[Title]
FROM [Posts] AS [p]
WHERE (
SELECT COUNT(*)
FROM [Comments] AS [c]
WHERE ([p].[PostId] = [c].[PostId]) AND ([c].[Likes] >= @likeThreshold)) > 0
)
```

The CLR method signature is as follows:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#QueryableFunctionDefinition)]

> [!TIP]
> The `FromExpression` call in the CLR function body allows for the function to be used instead of a regular DbSet.
And below is the mapping:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Model.cs#QueryableFunctionConfigurationHasDbFunction)]

> [!CAUTION]
> Until [issue 23408](https://github.com/dotnet/efcore/issues/23408) is fixed, mapping to an `IQueryable` of entity types overrides the default mapping to a table for the DbSet. If necessary - for example when the entity is not keyless - mapping to the table must be specified explicitly using `ToTable` method.
> [!NOTE]
> Queryable function must be mapped to a table-valued function and can't use of `HasTranslation`.
When the function is mapped, the following query:

[!code-csharp[Main](../../../samples/core/Querying/UserDefinedFunctionMapping/Program.cs#TableValuedFunctionQuery)]

Produces:

```sql
SELECT [p].[PostId], [p].[BlogId], [p].[Content], [p].[Rating], [p].[Title]
FROM [dbo].[PostsWithPopularComments](@likeThreshold) AS [p]
ORDER BY [p].[Rating]
```
2 changes: 2 additions & 0 deletions entity-framework/index.yml
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Expand Up @@ -151,6 +151,8 @@ additionalContent:
text: "Asynchronous queries"
- url: core/querying/raw-sql.md
text: "Raw SQL queries"
- url: core/querying/user-defined-function-mapping.md
text: "User-defined function mapping"
- url: core/querying/filters.md
text: "Global query filters"
# Card
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4 changes: 2 additions & 2 deletions entity-framework/toc.yml
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Expand Up @@ -185,8 +185,8 @@
href: core/querying/raw-sql.md
- name: Database functions
href: core/querying/database-functions.md
#- name: User defined functions
# href: core/querying/user-defined-functions.md
- name: User-defined function mapping
href: core/querying/user-defined-function-mapping.md
- name: Global query filters
href: core/querying/filters.md
- name: Query tags
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3 changes: 2 additions & 1 deletion samples/core/Modeling/Conventions/Conventions.csproj
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Expand Up @@ -8,7 +8,8 @@
</PropertyGroup>

<ItemGroup>
<PackageReference Include="Microsoft.EntityFrameworkCore.Relational" Version="3.1.8" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Relational" Version="5.0.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="5.0.0" />
</ItemGroup>

</Project>
26 changes: 26 additions & 0 deletions samples/core/Modeling/Conventions/EntityTypes.cs
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Expand Up @@ -38,4 +38,30 @@ public class AuditEntry
public string Action { get; set; }
}
#endregion

#region BlogWithMultiplePostsEntity
public class BlogWithMultiplePosts
{
public string Url { get; set; }
public int PostCount { get; set; }
}
#endregion

public class MyContextWithFunctionMapping : DbContext
{
public DbSet<Blog> Blogs { get; set; }
public DbSet<Post> Posts { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
#region QueryableFunctionConfigurationToFunction
modelBuilder.Entity<BlogWithMultiplePosts>().HasNoKey().ToFunction("BlogsWithMultiplePosts");
#endregion
}

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFModeling.EntityTypeToFunctionMapping;Trusted_Connection=True;ConnectRetryCount=0");
}
}
}
31 changes: 27 additions & 4 deletions samples/core/Modeling/Conventions/Program.cs
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@@ -1,15 +1,38 @@
using System;
using System.Collections.Generic;
using EFModeling.Conventions.EntityTypes;
using Microsoft.EntityFrameworkCore;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EFModeling.Conventions
{
class Program
{
static void Main(string[] args)
{
using (var context = new MyContextWithFunctionMapping())
{
context.Database.EnsureDeleted();
context.Database.EnsureCreated();

context.Database.ExecuteSqlRaw(
@"CREATE FUNCTION dbo.BlogsWithMultiplePosts()
RETURNS TABLE
AS
RETURN
(
SELECT b.Url, COUNT(p.BlogId) AS PostCount
FROM Blogs AS b
JOIN Posts AS p ON b.BlogId = p.BlogId
GROUP BY b.BlogId, b.Url
HAVING COUNT(p.BlogId) > 1
)");

#region ToFunctionQuery
var query = from b in context.Set<BlogWithMultiplePosts>()
where b.PostCount > 3
select new { b.Url, b.PostCount };
#endregion
var result = query.ToList();
}
}
}
}
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