I have a problem with MultiMaps in dapper trying to split on column that contains NULL
. Dapper seems not to instantiate object and my mapping function receives null
instead of object.
Here's my new test:
class Product
{
public int Id { get; set; }
public string Name { get; set; }
public Category Category { get; set; }
}
class Category
{
public int Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
}
public void TestMultiMapWithSplitWithNullValue()
{
var sql = @"select 1 as id, 'abc' as name, NULL as description, 'def' as name";
var product = connection.Query<Product, Category, Product>(sql, (prod, cat) =>
{
prod.Category = cat;
return prod;
}, splitOn: "description").First();
// assertions
product.Id.IsEqualTo(1);
product.Name.IsEqualTo("abc");
product.Category.IsNotNull();
product.Category.Id.IsEqualTo(0);
product.Category.Name.IsEqualTo("def");
product.Category.Description.IsNull();
}
The line that fails is product.Category.IsNotNull();
due to the fact that cat
passed to mapping function is null
.
I've also added this method to Assert class:
public static void IsNotNull(this object obj)
{
if (obj == null)
{
throw new ApplicationException("Expected not null");
}
}
This is "by-design" though I would be ok to revisit it.
In particular this behaviour is there to help with left joins. Take this for example:
cnn.Query<Car,Driver>("select * from Cars c left join Drivers on c.Id = CarId",
(c,d) => {c.Driver = d; return c;})
Trouble is that if we allow a "blanket" creation of a Driver
object, every Car
is going to have a Driver
even ones where the join failed.
To work around we could scan the entire segment being split and ensure ALL values are NULL
before mapping a NULL
object. This will have a very minor perf impact on the multi mapper.
To workaround for your case, you could insert a surrogate column:
var sql = @"select 1 as id, 'abc' as name, '' as split,
NULL as description, 'def' as name";
var product = connection.Query<Product, Category, Product>(sql, (prod, cat) =>
{
prod.Category = cat;
return prod;
}, splitOn: "split").First();
I have the same issue, i'm forced to select a fake 'split' columns to make Dapper fill my object instead of just null it;
My workaround :
string req = @"
SELECT
T1.a as PropA,
T1.b as PropB,
1 as Split,
T2.a as PropA,
T2.b as PropB,
1 as Split,
...
FROM
xxx T1,
yyy T2,
...";
using (var db = new OracleConnection(...))
{
return db.Query(
req,
(T1, T2) => {
...
},
splitOn:'Split,Split,...');
}
Dapper should have an option to avoid the splitOn:'Split,Split,...'