Composite Key Support

SqlMapperExtensions.cs

At the beginning of the method for GetAll, there is a GetSingleKey call within the method that seems a little unnecessary.

I have a situation, which is not uniques, where I don't have control over the data I am storing. This data needs a composite key to be unique.

I propose commenting out GetSingleKey(nameof(GetAll)) since the only thing being done in this method is a 'select * from tablename'

It might take more than just commenting out to create a fix.

The other option would be to allow for a single KeyAttribute and one or more ExplicitKeyAttributes in the GetSingleKey method.

`public static IEnumerable GetAll(this IDbConnection connection, IDbTransaction transaction = null, int? commandTimeout = null) where T : class
{
var type = typeof(T);
var cacheType = typeof(List);

        if (!GetQueries.TryGetValue(cacheType.TypeHandle, out string sql))
        {
            **GetSingleKey<T>(nameof(GetAll));**
            var name = GetTableName(type);

            sql = "select * from " + name;
            GetQueries[cacheType.TypeHandle] = sql;
        }

        if (!type.IsInterface()) return connection.Query<T>(sql, null, transaction, commandTimeout: commandTimeout);

        var result = connection.Query(sql);
        var list = new List<T>();
        foreach (IDictionary<string, object> res in result)
        {
            var obj = ProxyGenerator.GetInterfaceProxy<T>();
            foreach (var property in TypePropertiesCache(type))
            {
                var val = res[property.Name];
                if (val == null) continue;
                if (property.PropertyType.IsGenericType() && property.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>))
                {
                    var genericType = Nullable.GetUnderlyingType(property.PropertyType);
                    if (genericType != null) property.SetValue(obj, Convert.ChangeType(val, genericType), null);
                }
                else
                {
                    property.SetValue(obj, Convert.ChangeType(val, property.PropertyType), null);
                }
            }
            ((IProxy)obj).IsDirty = false;   //reset change tracking and return
            list.Add(obj);
        }
        return list;
    }

`

Start in SqlMapperExtensions.cs at GetAll and its GetSingleKey(nameof(GetAll)) call, then inspect how KeyAttribute and ExplicitKeyAttribute are handled. Determine the intended behavior for composite-key entities and verify that GetAll can retrieve them without inappropriate single-key validation; done means the chosen behavior is implemented and covered by the repository's existing tests.