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copied!<p>I've not been brave enough to try using NHibernate with F#'s type system, but it might help to look from the perspective of what's actually generated by the F# compiler.</p> <p>If you look at your Discriminated Union in reflector, there are actually three classes generated (and more if you count the private debug proxies).</p> <pre><code>public abstract class RequestInfo : IStructuralEquatable, IComparable, IStructuralComparable </code></pre> <p>The first class, RequestInfo, is abstract, and is actually implemented by the other types in the union.</p> <pre><code> // Nested Types [Serializable, DebuggerTypeProxy(typeof(Program.RequestInfo._Id@DebugTypeProxy)), DebuggerDisplay("{__DebugDisplay()}")] public class _Id : Program.RequestInfo { // Fields [DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode] public readonly int id1; // Methods [CompilerGenerated, DebuggerNonUserCode] public _Id(int id1); } [Serializable, DebuggerTypeProxy(typeof(Program.RequestInfo._Name@DebugTypeProxy)), DebuggerDisplay("{__DebugDisplay()}")] public class _Name : Program.RequestInfo { // Fields [DebuggerBrowsable(DebuggerBrowsableState.Never), CompilerGenerated, DebuggerNonUserCode] public readonly string name1; // Methods [CompilerGenerated, DebuggerNonUserCode] public _Name(string name1); } </code></pre> <p>so when you do:</p> <pre><code> let r=Id(5) let s=Name("bob") </code></pre> <p>r and s are instances of _Id and _Name, respectively.</p> <p>So the answer to your question is likely the answer to one of the following questions:</p> <ul> <li>How do I map to an abstract class in nhibernate?</li> <li>How can I make NHibernate use a factory method?</li> <li>How can I create map Nhibernate to immutable objects?</li> <li>How do I do implement a custom type in NHibernate (presumably with IUserType).</li> </ul> <p>Unfortunately, I'm not savvy enough to give you a coherent answer to any of those, but I'm sure someone else here has done at least one of these three solutions.</p> <p>I'd like to think that you can use the same methods used for Inheritance Strategies, using, for example, a discriminator column, but I'm afraid the lack of a default constructor makes this problematic. So I'm inclined to think that using a custom type is the solution.</p> <p>After some fiddling, here's a (possibly buggy and or broken) custom user type:</p> <pre><code>type RequestInfo = | Id of int | Name of string type RequestInfoUserType() as self = interface IUserType with member x.IsMutable = false member x.ReturnedType = typeof<RequestInfo> member x.SqlTypes = [| NHibernate.SqlTypes.SqlType(Data.DbType.String); NHibernate.SqlTypes.SqlType(Data.DbType.Int32); NHibernate.SqlTypes.SqlType(Data.DbType.String) |] member x.DeepCopy(obj) = obj //Immutable objects shouldn't need a deep copy member x.Replace(original,target,owner) = target // this might be ok member x.Assemble(cached, owner) = (x :> IUserType).DeepCopy(cached) member x.Disassemble(value) = (x :> IUserType).DeepCopy(value) member x.NullSafeGet(rs, names, owner)= // we'll use a column as a type discriminator, and assume the first mapped column is an int, and the second is a string. let t,id,name = rs.GetString(0),rs.GetInt32(1),rs.GetString(2) match t with | "I" -> Id(id) :> System.Object | "N" -> Name(name) :> System.Object | _ -> null member x.NullSafeSet(cmd, value, index)= match value with | :? RequestInfo -> let record = value :?> RequestInfo match record with | Id(i) -> cmd.Parameters.Item(0) <- "I" cmd.Parameters.Item(1) <- i | Name(n) -> cmd.Parameters.Item(0) <- "N" cmd.Parameters.Item(2) <- n | _ -> raise (new ArgumentException("Unexpected type")) member x.GetHashCode(obj) = obj.GetHashCode() member x.Equals(a,b) = if (Object.ReferenceEquals(a,b)) then true else if (a=null && b=null) then false else a.Equals(b) end </code></pre> <p>This code could surely be made more generic, and should probably not be in your actual domain layer, but I thought it would be useful to take a stab at a F# implementation of IUserType.</p> <p>Your mapping file would then do something like:</p> <pre><code><property name="IdOrName" type="MyNamespace.RequestInfoUserType, MyAssembly" > <column name="Type"/> <column name="Id"/> <column name="Name"/> </property> </code></pre> <p>You probably can get away without a column for "Type" with a slight tweak to the custom UserType code.</p> <p>I don't know how these custom user types work with queries/ICriteria, as I haven't really worked with custom user types much before.</p>

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