Reuse DTO mappings across both single and list variants - entity-framework

Question

We regularily write extension methods like this that convert from Database objects to DTO objects for use elsewhere in our system.
As you can see in the example below, the actual mapping code is repeated. Is it possible to write a reusable select mapping that can be used in both of these methods?
public static async Task<List<Group>> ToCommonListAsync(this IQueryable<DataLayer.Models.Group> entityGroups)
{
var groups =
await entityGroups.Select(
g =>
new Group()
{
Id = g.Id,
AccountId = g.AccountId,
Name = g.Name,
ParentId = g.ParentId,
UserIds = g.GroupUserMappings.Select(d => d.UserId).ToList()
}).ToListAsync();
return groups;
}
public static async Task<Group> ToCommonFirstAsync(this IQueryable<DataLayer.Models.Group> entityGroups)
{
var group =
await entityGroups.Select(
g =>
new Group()
{
Id = g.Id,
AccountId = g.AccountId,
Name = g.Name,
ParentId = g.ParentId,
UserIds = g.GroupUserMappings.Select(d => d.UserId).ToList()
}).FirstOrDefaultAsync();
return group;
}

Answer 1

You could move your mapping/projection code out into a variable like this:
public static class Extensions
{
private static readonly Expression<Func<DataLayer.Models.Group, Group>> Projection = g =>
new Group
{
Id = g.Id,
AccountId = g.AccountId,
Name = g.Name,
ParentId = g.ParentId,
UserIds = g.GroupUserMappings.Select(d => d.UserId).ToList()
};
public static async Task<List<Group>> ToCommonListAsync(this IQueryable<DataLayer.Models.Group> entityGroups)
{
return await entityGroups.Select(Projection).ToListAsync();
}
public static async Task<Group> ToCommonFirstAsync(this IQueryable<DataLayer.Models.Group> entityGroups)
{
return await entityGroups.Select(Projection).FirstOrDefaultAsync();
}
}