Resolving Issues with Dapper and Common Table Expressions: Column Mapping Solutions

Mapping CTE Rows with Dapper: Understanding the Issue and Possible Solutions

As a technical blogger, I’m here to help you understand why your SQL queries aren’t yielding the expected results when using Dapper for ORM purposes. In this article, we’ll delve into the world of Common Table Expressions (CTEs), column mapping, and how Dapper handles them.

Understanding CTEs

Common Table Expressions (CTEs) are temporary result sets that are defined within a SQL statement. They can be used to simplify complex queries or to create derived tables that can be referenced multiple times within a query.

Example of a Simple CTE

WITH CTE AS (
    SELECT value as group_description, kg.id AS group_id, kg.name AS name
    FROM keycloak_group kg
    JOIN group_attribute ga ON ga.group_id = kg.id AND ga.name = 'CompanyId'
    WHERE ga.value = '7bc5672e-9fb9-43f2-ab2e-da03dca0c32d'
)
SELECT * FROM CTE;

In this example, the CTE is used to simplify a complex query by breaking it down into smaller, more manageable pieces.

The Issue with Dapper

When using Dapper, you often need to map your SQL queries to classes or objects in your code. However, when working with CTEs, things can get tricky. As we’ll see later, the issue lies in how Dapper handles column names and mapping.

Query with CTE

var query = @"
    WITH t AS (
        SELECT
            (SELECT value from group_attribute ga2 WHERE ga2.group_id = kg.id AND ga2.name = 'description') as group_description,
            kg.id AS group_id,
            kg.name AS name,
            kg.parent_group AS parent_group,
            kg.realm_id AS realm_id,
            ga.id AS group_attr_id,
            ga.name AS group_attr_name,
            ga.value AS group_attr_value,
            ga.value AS group_attr_group_id
        FROM keycloak_group kg
                JOIN group_attribute ga ON ga.group_id = kg.id AND ga.name = 'CompanyId'
        WHERE ga.value = '7bc5672e-9fb9-43f2-ab2e-da03dca0c32d'
    UNION
    SELECT
        (SELECT value from group_attribute ga2 WHERE ga2.group_id = kg.id AND ga2.name = 'description') as group_description,
        kg.id AS group_id,
        kg.name AS name,
        kg.parent_group AS parent_group,
        kg.realm_id AS realm_id,
        ga.id AS group_attr_id,
        ga.name AS group_attr_name,
        ga.value AS group_attr_value,
        ga.value AS group_attr_group_id
    FROM keycloak_group kg
            JOIN group_attribute ga ON ga.group_id = kg.id
    WHERE  ga.name = 'isCommon' and ga.value = 'true')
SELECT DISTINCT COUNT(*) over (partition by t.group_id) as members_count, (SELECT DISTINCT COUNT(*) over (partition by 1) from t) as total_count,t.*
FROM t
JOIN user_group_membership ugm ON ugm.group_id = t.group_id;
";

var result = await connection.QueryAsync<RoleEntityV2>(query, new { tenantKey = request.TenantKey });

In this example, we have a query that uses a CTE to simplify the query. However, when using Dapper, the output doesn’t match our expectations.

The Problem with Column Mapping

The issue lies in how Dapper handles column mapping. When you use the [Column(...)] attribute on your POCO properties, Dapper maps those columns to the corresponding property names.

However, when working with CTEs, Dapper doesn’t map columns to their aliased names. Instead, it uses the original column names specified in the SQL query.

Example of Column Mapping

public class RoleEntityV2
{
    [Column("members_count")]
    public int MembersCount { get; set; }

    [Column("total_count")]
    public string TotalCount { get; set; }

    [Column("group_description")]
    public string Description { get; set; }
    
    [Column("group_id")]
    public Guid Id { get; set; }
    
    [Column("name")]
    public string Name { get; set; }
}

In this example, we have a POCO class RoleEntityV2 with properties mapped to columns using the [Column(...)] attribute. However, when working with CTEs, these column names don’t match the aliased names used in the SQL query.

Solution 1: Change the POCO

One solution is to change your POCO class so that the property names align with the original column names specified in the SQL query. This way, Dapper can map the columns correctly.

public class RoleEntityV2
{
    [Column("group_id")]
    public Guid Id { get; set; }

    [Column("name")]
    public string Name { get; set; }

    [Column("members_count")]
    public int MembersCount { get; set; }

    [Column("total_count")]
    public string TotalCount { get; set; }
}

In this example, we’ve changed the property names to match the original column names. Now, Dapper can map the columns correctly.

Solution 2: Change the Query

Another solution is to change the SQL query to include column aliases that match your POCO properties.

WITH t AS (
    SELECT
        (SELECT value from group_attribute ga2 WHERE ga2.group_id = kg.id AND ga2.name = 'description') as group_description,
        kg.id AS group_id,
        kg.name AS name,
        kg.parent_group AS parent_group,
        kg.realm_id AS realm_id,
        ga.id AS group_attr_id,
        ga.name AS group_attr_name,
        ga.value AS group_attr_value,
        ga.value AS group_attr_group_id
    FROM keycloak_group kg
            JOIN group_attribute ga ON ga.group_id = kg.id AND ga.name = 'CompanyId'
    WHERE ga.value = '7bc5672e-9fb9-43f2-ab2e-da03dca0c32d'
)
SELECT 
    COUNT(*) OVER (PARTITION BY t.group_id) AS members_count,
    COUNT(*) OVER () AS total_count,
    t.group_description
FROM t
JOIN user_group_membership ugm ON ugm.group_id = t.group_id;

In this example, we’ve changed the SQL query to include column aliases that match our POCO properties. Now, Dapper can map the columns correctly.

Conclusion

When working with Common Table Expressions (CTEs) and Dapper, it’s essential to understand how Dapper handles column mapping. By changing your POCO class or modifying your SQL query to include column aliases, you can resolve issues with CTEs not yielding expected results.

Last modified on 2023-06-18