Custom mapping in Dapper

Question

I'm attempting to use a CTE with Dapper and multi-mapping to get paged results. I'm hitting an inconvenience with duplicate columns; the CTE is preventing me from having to Name columns for example.

I would like to map the following query onto the following objects, not the mismatch between the column names and properties.

Query:

WITH TempSites AS(
    SELECT
        [S].[SiteID],
        [S].[Name] AS [SiteName],
        [S].[Description],
        [L].[LocationID],
        [L].[Name] AS [LocationName],
        [L].[Description] AS [LocationDescription],
        [L].[SiteID] AS [LocationSiteID],
        [L].[ReportingID]
    FROM (
        SELECT * FROM [dbo].[Sites] [1_S]
        WHERE [1_S].[StatusID] = 0
        ORDER BY [1_S].[Name]
        OFFSET 10 * (1 - 1) ROWS
        FETCH NEXT 10 ROWS ONLY
    ) S
        LEFT JOIN [dbo].[Locations] [L] ON [S].[SiteID] = [L].[SiteID]
),
MaxItems AS (SELECT COUNT(SiteID) AS MaxItems FROM Sites)

SELECT *
FROM TempSites, MaxItems

Objects:

public class Site
{
    public int SiteID { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public List<Location> Locations { get; internal set; }
}

public class Location
{
    public int LocationID { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public Guid ReportingID { get; set; }
    public int SiteID { get; set; }
}

For some reason I have it in my head that a naming convention exists which will handle this scenario for me but I can't find mention of it in the docs.

Answer 1

There are more than one issues, let cover them one by one.

CTE duplicate column names:

CTE does not allow duplicate column names, so you have to resolve them using aliases, preferably using some naming convention like in your query attempt.

For some reason I have it in my head that a naming convention exists which will handle this scenario for me but I can't find mention of it in the docs.

You probably had in mind setting the DefaultTypeMap.MatchNamesWithUnderscores property to true, but as code documentation of the property states:

Should column names like User_Id be allowed to match properties/fields like UserId?

apparently this is not the solution. But the issue can easily be solved by introducing a custom naming convention, for instance "{prefix}{propertyName}" (where by default prefix is "{className}_") and implementing it via Dapper's CustomPropertyTypeMap. Here is a helper method which does that:

public static class CustomNameMap
{
    public static void SetFor<T>(string prefix = null)
    {
        if (prefix == null) prefix = typeof(T).Name + "_";
        var typeMap = new CustomPropertyTypeMap(typeof(T), (type, name) =>
        {
            if (name.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
                name = name.Substring(prefix.Length);
            return type.GetProperty(name);
        });
        SqlMapper.SetTypeMap(typeof(T), typeMap);
    }
}

Now all you need is to call it (one time):

CustomNameMap.SetFor<Location>();

apply the naming convention to your query:

WITH TempSites AS(
    SELECT
        [S].[SiteID],
        [S].[Name],
        [S].[Description],
        [L].[LocationID],
        [L].[Name] AS [Location_Name],
        [L].[Description] AS [Location_Description],
        [L].[SiteID] AS [Location_SiteID],
        [L].[ReportingID]
    FROM (
        SELECT * FROM [dbo].[Sites] [1_S]
        WHERE [1_S].[StatusID] = 0
        ORDER BY [1_S].[Name]
        OFFSET 10 * (1 - 1) ROWS
        FETCH NEXT 10 ROWS ONLY
    ) S
        LEFT JOIN [dbo].[Locations] [L] ON [S].[SiteID] = [L].[SiteID]
),
MaxItems AS (SELECT COUNT(SiteID) AS MaxItems FROM Sites)

SELECT *
FROM TempSites, MaxItems

and you are done with that part. Of course you can use shorter prefix like "Loc_" if you like.

Mapping the query result to the provided classes:

In this particular case you need to use the Query method overload that allows you to pass Func<TFirst, TSecond, TReturn> map delegate and unitilize the splitOn parameter to specify LocationID as a split column. However that's not enough. Dapper's Multi Mapping feature allows you to split a single row to a several single objects (like LINQ Join) while you need a Site with Location list (like LINQ GroupJoin).

It can be achieved by using the Query method to project into a temporary anonymous type and then use regular LINQ to produce the desired output like this:

var sites = cn.Query(sql, (Site site, Location loc) => new { site, loc }, splitOn: "LocationID")
    .GroupBy(e => e.site.SiteID)
    .Select(g =>
    {
        var site = g.First().site;
        site.Locations = g.Select(e => e.loc).Where(loc => loc != null).ToList();
        return site;
    })
    .ToList();

where cn is opened SqlConnection and sql is a string holding the above query.

Answer 2

You can map a column name with another attribute using the ColumnAttributeTypeMapper.

See my first comment on the Gist for further details.

You can do the mapping like

public class Site
{
    public int SiteID { get; set; }
    [Column("SiteName")]
    public string Name { get; set; }
    public string Description { get; set; }
    public List<Location> Locations { get; internal set; }
}

public class Location
{
    public int LocationID { get; set; }
    [Column("LocationName")]
    public string Name { get; set; }
    [Column("LocationDescription")]
    public string Description { get; set; }
    public Guid ReportingID { get; set; }
    [Column("LocationSiteID")]
    public int SiteID { get; set; }
}

Mapping can be done using either of the following 3 methods

Method 1

Manually set the custom TypeMapper for your Model once as:

Dapper.SqlMapper.SetTypeMap(typeof(Site), new ColumnAttributeTypeMapper<Site>());
Dapper.SqlMapper.SetTypeMap(typeof(Location), new ColumnAttributeTypeMapper<Location>());

Method 2

For class libraries of .NET Framework >= v4.0, you can use PreApplicationStartMethod to register your classes for custom type mapping.

using System.Web;
using Dapper;

[assembly: PreApplicationStartMethod(typeof(YourNamespace.Initiator), "RegisterModels")]

namespace YourNamespace
{
    public class Initiator
    {
        private static void RegisterModels()
        {
             SqlMapper.SetTypeMap(typeof(Site), new ColumnAttributeTypeMapper<Site>());
             SqlMapper.SetTypeMap(typeof(Location), new ColumnAttributeTypeMapper<Location>());
             // ...
        }
    }
}

Method 3

Or you can find the classes to which ColumnAttribute is applied through reflection and set type mappings. This could be a little slower, but it does all the mappings in your assembly automatically for you. Just call RegisterTypeMaps() once your assembly is loaded.

    public static void RegisterTypeMaps()
    {
        var mappedTypes = Assembly.GetAssembly(typeof (Initiator)).GetTypes().Where(
            f =>
            f.GetProperties().Any(
                p =>
                p.GetCustomAttributes(false).Any(
                    a => a.GetType().Name == ColumnAttributeTypeMapper<dynamic>.ColumnAttributeName)));

        var mapper = typeof(ColumnAttributeTypeMapper<>);
        foreach (var mappedType in mappedTypes)
        {
            var genericType = mapper.MakeGenericType(new[] { mappedType });
            SqlMapper.SetTypeMap(mappedType, Activator.CreateInstance(genericType) as SqlMapper.ITypeMap);
        }
    }