Using Dapper to map more than 5 types

问题:

I am currently building a SELECT query that joins 12 tables together. I've been using Dapper for all my other queries and it works great. Problem is, the generic methods only have to five generic parameters.

I've previously modified the code to support up to 6 for another query, but now I really don't think I should be hacking 6 more levels of generics.

Is there a way to pass dapper an array of types, and it returns the results as an array of objects, which I can cast manually if I have to?

I also might be approaching the problem the wrong way! Any help will be appreciated!

回答1:

In a project I worked on I saw something like this to get more than 7 types mapped. We used Dapper 1.38:

connection.Query<TypeOfYourResult>
(
   queryString,
   new[]
   {
      typeof(TypeOfArgument1),
      typeof(TypeOfArgument2),
      ...,
      typeof(TypeOfArgumentN)
   },
   objects =>
   {
      TypeOfArgument1 arg1 = objects[0] as TypeOfArgument1;
      TypeOfArgument2 arg2 = objects[1] as TypeOfArgument2;
      ...
      TypeOfArgumentN argN = objects[N] as TypeOfArgumentN;

     // do your processing here, e.g. arg1.SomeField = arg2, etc.
     // also initialize your result

     var result = new TypeOfYourResult(...)

     return result;
   },
   parameters,
   splitOn: "arg1_ID,arg2_ID, ... ,argN_ID"
);

The queryString is self-explanatory. The splitOn parameter says how Dapper should split the columns from the SELECT statement so that everything can be mapped properly to the objects, you can read about it here.

回答2:

You could use a dynamic query and map it afterwards. Something like this

var result = conn.Query<dynamic>(query).Select(x => new Tuple<Type1, Type2, Type3, Type4, Type5>( 
// type initialization here 
    new Type1(x.Property1,x.Property2),
    new Type2(x.Property3,x.Property4),
    new Type3(x.Property5,x.Property6) etc....));

Edit: With a rather huge result set, another option might be to use multiple querys and then use a Grid Reader. That might work for you.

There's the example taken from the dapper age:

var sql = 
@"
select * from Customers where CustomerId = @id
select * from Orders where CustomerId = @id
select * from Returns where CustomerId = @id";

using (var multi = connection.QueryMultiple(sql, new {id=selectedId}))
{
   var customer = multi.Read<Customer>().Single();
   var orders = multi.Read<Order>().ToList();
   var returns = multi.Read<Return>().ToList();
   ...
} 

回答3:

This has been answered long time ago, but I would like to add my two cents here. Instead of manually modify Dapper's source code, why don't you just create a poco class with those fields and use your query like a table?

The mapping would work fine, I know it is a pain also to do that class definition, but seems easier than dealing with later Dapper's updates.