Entity Framework, Linq : concatenating results from child table - entity-framework

Question

I have an existing linq query which gets some data into a view model object. This is working fine.
I want to add a new property for data from child table which will have column values from a child table in a comma separated string format.
Problem: I am not able to concatenate the results using string.join
Simplified version of tables showing only relevant fields
part
id
part number
1
ABC1
2
DEF1
vendor
id
vendorname
1
acme
2
john
vendor part name (vendor specific part number)
partid
vendorid
partname
1
1
GDSE-553-32
1
2
JWWVV-HH-01
simplified version of query
result = (from p in DBContext.Parts.Where(w => w.EquipmentId == eId)
select new PartModel
{
Id = p.Id,
Number = p.PartNumber,
VendorPartNames= String.Join(",", DBContext.VendorPartName.Where(w => w.PartId == p.Id).Select(s => s.PartName))//this line causes exception (shown below)
});
Exception:
LINQ to Entities does not recognize the method 'System.String Join(System.String, System.String[])' method, and this method cannot be translated into a store expression.
Please note: the actual query has some joins and other columns, so please dont suggest solutions that requires joins.
If I change the "VendorPartName" to a List type , I can get the results without any problems.
My only problem is in "How to convert the results for "VendorPartName" property to a comma separated strings?"
eg: based on sample table data provided, it should be
GDSE-553-32, JWWVV-HH-01

Answer 1

Entity Framework does not support String.Join() method.
So, what we can do is to fetch VendorPartNames as a string collection and then we can later separate it with ,.
Note: For this, we would first use an anonymous object and later convert it to PartModel.
So your query would look like this:
var parts = DBContext.Parts
.Where(w => w.EquipmentId == eId)
.Select(p => new {
Id = p.Id,
Number = p.PartNumber,
VendorPartNames = p.VendorPartName.Select(n => n.PartName)
}).ToList();
var result = parts.Select(i => new PartModel {
Id = i.Id,
Number = i.Number,
VendorPartNames = String.Join(",", i.VendorPartNames)
}).ToList();