Using Automapper with F# Entity Type Provider - entity-framework

Question

I am looking at Automapper for the first time using F# and the Entity Type Provider. I want to map between the EF Type Provider types and the F# record types that I have created.
The EF Type Provider is based on the following database schema:
CREATE TABLE [dbo].[Address](
[Id] [int] IDENTITY(1,1) NOT NULL,
[FamilyId] [int] NOT NULL,
[State] [varchar](50) NOT NULL,
[County] [varchar](50) NOT NULL,
[City] [varchar](50) NOT NULL,
CONSTRAINT [PK_Address] PRIMARY KEY CLUSTERED ([Id] ASC)
CREATE TABLE [dbo].[Child](
[Id] [int] IDENTITY(1,1) NOT NULL,
[FamilyId] [int] NOT NULL,
[FirstName] [varchar](50) NOT NULL,
[Gender] [varchar](50) NOT NULL,
[Grade] [int] NOT NULL,
CONSTRAINT [PK_Child] PRIMARY KEY CLUSTERED ([Id] ASC)
CREATE TABLE [dbo].[Family](
[Id] [int] IDENTITY(1,1) NOT NULL,
[LastName] [varchar](50) NOT NULL,
[IsRegistered] [bit] NOT NULL,
CONSTRAINT [PK_Family] PRIMARY KEY CLUSTERED ([Id] ASC)
CREATE TABLE [dbo].[Parent](
[Id] [int] IDENTITY(1,1) NOT NULL,
[FamilyId] [int] NOT NULL,
[FirstName] [varchar](50) NOT NULL,
CONSTRAINT [PK_Parent] PRIMARY KEY CLUSTERED ([Id] ASC)
CREATE TABLE [dbo].[Pet](
[Id] [int] IDENTITY(1,1) NOT NULL,
[ChildId] [int] NOT NULL,
[GivenName] [varchar](50) NOT NULL,
CONSTRAINT [PK_Pet] PRIMARY KEY CLUSTERED ([Id] ASC)
ALTER TABLE [dbo].[Address] WITH CHECK ADD CONSTRAINT [FK_Address_Family] FOREIGN KEY([FamilyId])
REFERENCES [dbo].[Family] ([Id])
ALTER TABLE [dbo].[Address] CHECK CONSTRAINT [FK_Address_Family]
ALTER TABLE [dbo].[Child] WITH CHECK ADD CONSTRAINT [FK_Child_Family] FOREIGN KEY([FamilyId])
REFERENCES [dbo].[Family] ([Id])
ALTER TABLE [dbo].[Child] CHECK CONSTRAINT [FK_Child_Family]
ALTER TABLE [dbo].[Parent] WITH CHECK ADD CONSTRAINT [FK_Parent_Family] FOREIGN KEY([FamilyId])
REFERENCES [dbo].[Family] ([Id])
ALTER TABLE [dbo].[Parent] CHECK CONSTRAINT [FK_Parent_Family]
ALTER TABLE [dbo].[Pet] WITH CHECK ADD CONSTRAINT [FK_Pet_Child] FOREIGN KEY([ChildId])
REFERENCES [dbo].[Child] ([Id])
ALTER TABLE [dbo].[Pet] CHECK CONSTRAINT [FK_Pet_Child]
I then created a comparable set of types in F#:
type Pet = {Id:int; GivenName:string}
type Child = {Id:int; FirstName:string; Gender:string; Grade:int; Pets: Pet list}
type Address = {Id:int; State:string; County:string; City:string}
type Parent = {Id:int; FirstName:string}
type Family = {Id:int; Parents:Parent list; Children: Child list; Address:Address}
The only real difference is that the foreign key is not explicit in the record types.
When I use Automapper on the Address type, it works as expected:
Mapper.CreateMap<EntityConnection.ServiceTypes.Address, Address>()
let context = EntityConnection.GetDataContext()
let addressQuery = query {for address in context.Addresses do select address}
let address = Seq.head addressQuery
let address' = Mapper.Map<Address>(address)
val address' : Address = {Id = 1;
State = "WA";
County = "King";
City = "Seattle";}
But when I try and do the same with the entire graph,
Mapper.CreateMap<EntityConnection.ServiceTypes.Pet, Pet>()
Mapper.CreateMap<EntityConnection.ServiceTypes.Child, Child>()
Mapper.CreateMap<EntityConnection.ServiceTypes.Address, Address>()
Mapper.CreateMap<EntityConnection.ServiceTypes.Parent, Parent>()
Mapper.CreateMap<EntityConnection.ServiceTypes.Family, Family>()
let context = EntityConnection.GetDataContext()
let familyQuery = query {for family in context.Families do select family}
let family = Seq.head familyQuery
let family' = Mapper.Map<Family>(family)
I get this exception:
System.ArgumentException: Type needs to have a constructor with 0 args or only optional args
Parameter name: type
I am wondering if it is b/c EF is lazy loading so the remaining types are not being evaluated? has anyone seen this before?

Answer 1

The error is pretty straight forward. None of your classes have a constructor that takes 0 arguments.
F# creates default constructors for you so the default constructor on your class has multiple arguments in it. For example:
type Pet = {Id:int; GivenName:string}
as a c# class would have this as it's definition.
public class Pet
{
public int Id { get; private set; }
public string GivenName { get; private set; }
public Pet(int id, string givenName)
{
Id = id;
GivenName = givenName;
}
}
Note the lack of a parameterless constructor. That's where your error is coming from.
You can fix it by flagging your type as CLIMutable
[<CLIMutable>]
type Pet = {Id:int; GivenName:string}