Dapper receive List as object parameter from JOIN query

Question

I have trouble using Dapper when i want to map my object from database.

class Set
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public List<Guid> GeneratorsList { get; set; }
}

It contains a list so I've created separated table that holds it, like that

CREATE TABLE [dbo].[Sets] (
    [Id]          UNIQUEIDENTIFIER NOT NULL,
    [Name]        VARCHAR (60)     NOT NULL,
    [Description] VARCHAR (60)     NULL,
    PRIMARY KEY CLUSTERED ([Id] ASC)
);

CREATE TABLE [dbo].[Set_Generator] (
    [SetId]       UNIQUEIDENTIFIER NOT NULL,
    [GeneratorId] UNIQUEIDENTIFIER NOT NULL,
    PRIMARY KEY CLUSTERED ([SetId] ASC, [GeneratorId] ASC),
    FOREIGN KEY ([SetId]) REFERENCES [dbo].[Sets] ([Id]) ON DELETE CASCADE
);

And from those tables I want to retrieve correct Set object with minimum code effort. I've tried to do join on them and strong type the result to Set, but with no success.

var result = conn.Query<Set>(@"SELECT [S].[Id], [S].[Name], [S].[Description], [SG].[GeneratorId] AS [GeneratorsList] FROM [Sets] AS [S] INNER JOIN [Set_Generator] AS [SG] ON [S].[Id] = [SG].[SetId];");

I'm aware that I could simply retrieve Set from table and then add the GeneratorsList from separate query, but I'm looking for a better solution.

var sets = conn.Query<Set>(@"SELECT * FROM [Sets];");
foreach(var set in sets)
{
    var generators = conn.Query<Guid>(@"SELECT [GeneratorId] FROM [Set_Generator] WHERE [SetId]=@SetId", new { SetId = set.Id });
    set.GeneratorsList = generators.ToList();
}

Is there a solution that require only one query ?

Use QueryMultiple stackoverflow.com/questions/6751052/…

Ric
– Ric

2017-09-06 13:06:09 +00:00
Commented Sep 6, 2017 at 13:06 — Ric
– Ric, Commented Sep 6, 2017 at 13:06

Artem · Accepted Answer · 2017-09-06 13:15:08Z

2

Using the query you provided you may receive:

internal class GeneratorSet
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public Guid GeneratorId { get; set; }
}

And use linq GroupBy to construct the Set object(-s):

var generatorSets = conn.Query<GeneratorSet>(@"SELECT [S].[Id], [S].[Name], [S].[Description], [SG].[GeneratorId] AS [GeneratorsList] FROM [Sets] AS [S] INNER JOIN [Set_Generator] AS [SG] ON [S].[Id] = [SG].[SetId];");;

var res = generatorSets.GroupBy(gen => gen.Id, p => p, (key, g) => new Set()
      { Id = key, Generators = g.Select(g => g.GeneratorId)});

answered Sep 6, 2017 at 13:15

Artem

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