OData GroupBy and Select - odata

Question

Expecting the following example table CustomerOrders
Id
CustomerId
Customer
Product
1
1
Alice
Pizza
2
1
Alice
Pasta
3
2
Bob
Burger
In C# I'm was able to use the following Linq query to produce a nice List<Customer> result with a nested orders collection for every customer:
List<CustomerOrders> queryResult = GetCustomerOrders();
return queryResult
.GroupBy(x => x.CustomerId)
.Select(x => new Customer
{
Id = x.First().CustomrId,
Customer = x.First().Customer,
Orders = x.ToList()
})
.ToList();
Now I want to achive this result directly over an odata query in the client application to get the following JSON result:
[
{
"id": 1,
"customer": Alice,
"orders": [ "Pizza", "Pasta" ]
},
{
"id": 2,
"customer": Bob,
"orders": [ "Burger" ]
}
]
Is there a way to transfer this query in odata?

Answer 1

GroupBy in OData is similar to SQL, only the aggregates and common columns are returned, we lose access to the individual items, so we can return a grouping and a count of the orders, using group by, but not the array of orders.
If your schema has a Customer entity and there is a collection navigation property from Customer to Orders, then we do not need to use grouping at all:
~/Customers?$expand=Orders($select=Product)&$select=Id,Name
The output is structured in a slightly similar manner and should resemble something like this:
{
"#odata.context": "~/$metadata#Customers(Id,Name,Orders(Product))",
"value": [
{
"Id": 1,
"Name": "Alice",
"Orders": [{"Product": "Pizza"},
{"Product": "Pasta"}]
},
{
"Id": 2,
"Name": "Bob",
"Orders": [{"Product": "Burger"}]
}
]
}
A key concept in OData is that the shape of the overall graph should not be modified, it is designed deliberately to always maintain the structure of the Entities that are returned. This means that the definition document is always correct, the only thing missing from this response is the additional fields that were not requested.
If you need the output in the client specifically as mentioned, then you can expose that as a custom function on the controller:
[EnableQuery]
public IQueryable<CustomerSummary> GetCustomersWithOrderSummary()
{
List<CustomerOrders> queryResult = GetCustomerOrders();
return queryResult
.GroupBy(x => x.CustomerId)
.Select(x => new CustomerSummary
{
Id = x.Key,
Customer = x.First().Customer,
Orders = x.Select(o => o.Product)
});
}
If using GroupBy, the closest response we can get is this:
~/CustomerOrders?$apply=groupby((CustomerId,Customer),aggregate($count as Orders))
But here we will return a count of the orders, and not an array of the product values as expected:
{
"#odata.context": "~/$metadata#CustomerOrders(CustomerId,Customer,Orders)",
"value": [
{
"#odata.id": null,
"CustomerId": 1,
"Customer": "Alice",
"Orders": 2
},
{
"#odata.id": null,
"CustomerId": 2,
"Customer": "Bob",
"Orders": 1
}
]
}