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 Na

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(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();

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 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.