What did you do?

I've created the following models:

[TableName("udfUnionMunicipalities")]
[PrimaryKey("id", autoIncrement = false)]
[ExplicitColumns]
public class UnionMunicipality
{
    /// <summary>
    /// OfficialCode
    /// </summary>
    [Column("id")]
    [PrimaryKeyColumn(AutoIncrement = false)]
    public string Id { get; set; }

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

/// <summary>
/// Company Category
/// </summary>
[TableName("udfUnionCategories")]
[PrimaryKey("id", autoIncrement = false)]
[ExplicitColumns]
public class UnionCategory
{
    [PrimaryKeyColumn(AutoIncrement = false)]
    [Column("id")]
    public int Id { get; set; }

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

/// <summary>
/// Company
/// </summary>
[TableName("udfUnions")]
[PrimaryKey("id", autoIncrement = false)]
[ExplicitColumns]
public class Union
{
    [Column("id")]
    [PrimaryKeyColumn(AutoIncrement = false)]
    public string Id { get; set; }

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

    [Column("categoryId")]
    [ForeignKey(typeof(UnionCategory))]
    public int CategoryId { get; set; }

    [ForeignKey(typeof(UnionMunicipality))]
    [Column("municipalityId")]
    public string MunicipalityId { get; set; }

    [ResultColumn]
    public UnionMunicipality Municipality { get; set; }

    [ResultColumn]
    public UnionCategory Category { get; set; }
}

Then i used the following query to join Category and Municipality on Union table

var sql = new Sql()
            .Select("udfUnions.*, udfUnionMunicipalities.*, udfUnionCategories.*")
            .From<Union>(dbContext.SqlSyntax);

            sql.InnerJoin<UnionMunicipality>(dbContext.SqlSyntax)
               .On<Union, UnionMunicipality>(dbContext.SqlSyntax, u => u.CategoryId, c => c.Id)
               .InnerJoin<UnionCategory>(dbContext.SqlSyntax)
               .On<Union, UnionCategory>(dbContext.SqlSyntax, u => u.CategoryId, c => c.Id);

            LogHelper.Info<UnionsRepository>(() => sql.SQL);

            return db.Fetch<Union, UnionMunicipality, UnionCategory, Union>((u, m, c) => {
               u.Municipality = m;
               u.Category = c;
               return u;
            }, sql);

Outputs following sql:

SELECT udfUnions.*, udfUnionMunicipalities.*, udfUnionCategories.*
FROM [udfUnions]
LEFT JOIN [udfUnionMunicipalities]
ON [udfUnions].[municipalityId] = [udfUnionMunicipalities].[id]
LEFT JOIN [udfUnionCategories]
ON [udfUnions].[categoryId] = [udfUnionCategories].[id]

I also tried writing the SQL manually like so:

SELECT *
FROM udfUnions
INNER JOIN udfUnionMunicipalities
ON udfUnions.MunicipalityId = udfMunicipalties.Id
INNER JOIN udfUnionCategories
ON udfUnions.CategoryId = udfUnionCategories.Id

Like the heading says, i also tried using LEFT JOIN

What did you expect to happen? That the tables would be mapped to correct result columns

What actually happened? I know it not much, but this is what the log shows:

System.NullReferenceException: Object reference not set to an instance of an object.

Any help would be appreciated!