My ADO.Net Entity Data Model has a model called ABC below (modeled after a self-referencing table).
ABC Properties are
----------
ParentID
Child ID
ABC Navigation Properties are
----------
ParentCategory (refers to the parent category or the 0..1 side of the relationship)
SubCategories (refers to the children or the * side of the relationship, represents the navigation property for the children)
I want to select the children and grandchildren for a specific ParentID (i.e. not the top of the hierarchy ). How can I accomplish this. Can someone please propose an example. Thanks
I have tried the solution proposed below in vb, but it is only loading one level;
I am doing this in VB so apologies to C# programmers.
Protected Sub Button2_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button2.Click
Dim ctx = New links2Entities
Dim query = From c In ctx.PBS.Include("SubCategory.SubCategory") Where (c.Parent_ID = 7)
For Each result As PB In query
Debug.WriteLine("ID: {0}", result.Child_ID)
Next
End Sub
If you need to select entity by its Id and eager load two levels of its children you need to do:
var query = context.ABC.Include("SubCategories.SubCategories")
.Where(e => e.Id == id);
In case of EF 4.1 it should be:
var query = context.ABS.Include(e => e.SubCategories.Select(s => s.SubCategories))
.Where(e => e.Id == id);
If you need to eager load all sub categories in any depth there is not way to tell it EF.
Related
Suppose the following tables
ParentEntities
ParentID
ChildEntities
ChildID
ParentID
These tables do not have a FK defined in the schema.
In EF designer, after generating from DB, I add an association:
- Parent Multiplicity: 1
- Child Multiplicity: 0 or 1
When I build, I get the error: "Error 3027: No mapping specified for the following EntitySet/AssociationSet - ParentChild"
But if I try to configure table mapping for the association like this..
Maps to ChildEntities
Parent
ParentID <-> ParentID (parent entity prop <-> child table column)
Child
ChildID <-> ChildID (child entity prop <-> child table column)
.. I get this: Error 3007: Problem in mapping fragments starting at lines xxx, xxx: Column(s) [ParentID] are being mapped in both fragments to different conceptual side properties.
Why this is an error doesn't make sense. Limitation of the current implementation?
[EDIT 1]
I'm able to make this work by creating a 1-N association. That's not ideal, but it works just the same, just have to add a read-only child property in a partial:
public partial class Parent
{
public Child Child { get { return Childs.Any() ? null : Childs.First(); } }
}
This seems like the best solution for me. I had to add a FK to the database to get EF to generate the association and navigation property, but once it was added I was able to remove the FK, and further updates to the model from the DB did not remove the association or Navigation properties.
[EDIT 2]
As I was investigating how to work around not caring about the association being modeled in EF, I ran into another issue. Instead of the read-only Child property I made it normal ..
public partial class Parent
{
public Child Child { get; set; }
}
.. but now I need a way to materialize that from the query:
var query = from parents in context.Parents
// pointless join given select
join child in context.Childs
on parents.ParentID equals child.ParentID
select parents;
I can select an anonymous type ..
// step 1
var query = from parents in context.Parents
join child in context.Childs
on parents.ParentID equals child.ParentID
select new { Parent = parents, Child = child };
.. but then I've got to consume more cycles getting that into my entity:
// step 2
var results = query.Select(x => {
var parent = x.Parent;
parent.Child = x.Child;
return parent; });
Is there a better/streamlined way to do this from the query select so the EF materializer can do it from the get-go? If not, then I'll resort to Edit 1 methodology ..
Ef Code first requires 1->0..1 relationships for the Child to have the same primary key.
Maybe this a similar restriction In the modeler in this circumstance.
ParentId (Key) required in Both tables.
I have never tried adding such relationships in designer afterwords in DB first.
EDIT: to match your EDIT2:
I would stay on the direction . Use Navigation properties to get from Join back to original class A and B.
query = context.Set<JoinTable>.Where(Jt=>Jt.NavA.Id == ClassAId
&& Jt.navB.Id == ClassBId)
use a select if your need entries returned from either ClassA or ClassB.
This question already has answers here:
EF: Include with where clause [duplicate]
(5 answers)
Closed 1 year ago.
It seems that I can't get a parent and a subset of its children from the db.
For example...
db.Parents
.Include(p => p.Children)
.Where(p => p.Children.Any(c => c.Age >= 5))
This will return all Parents that have a child aged 5+, but if I iterate through the Parents.Children collection, all children will be present (not just those over 5 years old).
Now the query does make sense to me (I've asked to include children and I've got them!), but can imagine that I would like to have the where clause applied to the child collection in some scenarios.
How could I get an IEnumerable, in which each of the parents has a filtered collection of Children (Age>=5)?
The only way to get a collection of parents with a filtered children collection in a single database roundtrip is using a projection. It is not possible to use eager loading (Include) because it doesn't support filtering, Include always loads the whole collection. The explicite loading way shown by #Daz requires one roundtrip per parent entity.
Example:
var result = db.Parents
.Select(p => new
{
Parent = p,
Children = p.Children.Where(c => c.Age >= 5)
})
.ToList();
You can directly work with this collection of anonymous type objects. (You can also project into your own named type instead of an anonymous projection (but not into an entity like Parent).)
EF's context will also populate the Children collection of the Parent automatically if you don't disable change tracking (using AsNoTracking() for example). In this case you can then project the parent out of the anonymous result type (happens in memory, no DB query):
var parents = result.Select(a => a.Parent).ToList();
parents[i].Children will contain your filtered children for each Parent.
Edit to your last Edit in the question:
I am after a) A list of parents who have a child older than 5 (and
include only those children).
The code above would return all parents and include only the children with Age >= 5, so potentially also parents with an empty children collection if there are only children with Age < 5. You can filter these out using an additional Where clause for the parents to get only the parents which have at least one (Any) child with Age >= 5:
var result = db.Parents
.Where(p => p.Children.Any(c => c.Age >= 5))
.Select(p => new
{
Parent = p,
Children = p.Children.Where(c => c.Age >= 5)
})
.ToList();
In EF Core 5.0, the Include method now supports filtering of the entities included.
https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-5.0/whatsnew#filtered-include
var data = db.Parents
.Include(p => p.Children.Where(c => c.Age >= 5))
.ToList();
Taking your example the following should do what you need. Take a look here for more info.
db.Entry(Parents)
.Collection("Children")
.Query().Cast<Child>()
.Where(c => c.Age >= 5))
.Load();
I think parents and child are not really well suited as separate entities. A child can always also be a parent and usually a child has two parents (a father and a mother), so it's not the simplest context. But I assume you just have a simple 1:n relationship as in the following master-slave model that I used.
What you need to do is make a left outer join (that answer has led me on the right path). Such a join is a bit tricky to do, but here's the code
var query = from m in ctx.Masters
join s in ctx.Slaves
on m.MasterId equals s.MasterId into masterSlaves
from ms in masterSlaves.Where(x => x.Age > 5).DefaultIfEmpty()
select new {
Master = m,
Slave = ms
};
foreach (var item in query) {
if (item.Slave == null) Console.WriteLine("{0} owns nobody.", item.Master.Name);
else Console.WriteLine("{0} owns {1} at age {2}.", item.Master.Name, item.Slave.Name, item.Slave.Age);
}
This will translate to the following SQL statement with EF 4.1
SELECT
[Extent1].[MasterId] AS [MasterId],
[Extent1].[Name] AS [Name],
[Extent2].[SlaveId] AS [SlaveId],
[Extent2].[MasterId] AS [MasterId1],
[Extent2].[Name] AS [Name1],
[Extent2].[Age] AS [Age]
FROM [dbo].[Master] AS [Extent1]
LEFT OUTER JOIN [dbo].[Slave] AS [Extent2]
ON ([Extent1].[MasterId] = [Extent2].[MasterId]) AND ([Extent2].[Age] > 5)
Note that it is important to perform the additional where clause on the age on the joined collection and not between the from and the select.
EDIT:
IF you want a hierarchical result you can convert the flat list by performing a grouping:
var hierarchical = from line in query
group line by line.Master into grouped
select new { Master = grouped.Key, Slaves = grouped.Select(x => x.Slave).Where(x => x != null) };
foreach (var elem in hierarchical) {
Master master = elem.Master;
Console.WriteLine("{0}:", master.Name);
foreach (var s in elem.Slaves) // note that it says elem.Slaves not master.Slaves here!
Console.WriteLine("{0} at {1}", s.Name, s.Age);
}
Note that I used an anonymous type to store the hierarchical result. You can of course create also a specific type like this
class FilteredResult {
public Master Master { get; set; }
public IEnumerable<Slave> Slaves { get; set; }
}
and then project the group into instances of this class. That makes it easier if you need to pass these results to other methods.
I have a many to many relationship in EF4 with the following entities:
[Student] - [Class] - [Student_Class]
Moreover i have a [School] entity with a FK on [Student].
If i want to have all the Students of my school i do:
context.School.Include("Student")
but if i want to have the 1rst class of my Students in my school ?
context.School.Include("Student").Include("Student_Class").Where(...
i did not manage to make this thing work...
Can you help ?
Also is it more intelligent to write a full Linq select?
Thanks
John
If you want to do a conditional eager load, then you should NOT be using the Include method.
For loading your school object containing only the students that belong to the first class
You can do a Filtered Projection which returns an Anonymous Type object:
var school = context.School
.Where(s => s.SchoolID == 1) // or any other predicate
.Select(s => new
{
School = s,
Students = s.Student.Where(st => st.ClassID == 1)
}).ToList();
Another way would be to Leverage Attach Method which returns EntityObject:
var school = context.School.Where(s => s.SchoolID == 1).First()
var sourceQuery = school.Students.CreateSourceQuery()
.Where(st => st.ClassID == 1);
school.Students.Attach(sourceQuery);
For a more detailed discussion about this, you can also check:
Entity Framework: How to query data in a Navigation property table
I would like to add a record to a SQL Server table using the Entity Framework. My table's entity has foreign keys and so has navigational properties for those fields. When adding a new record/entity, how do I populate the foreign key fields since they don't appear as properties of the entity?
The easiest way is to do a query for the related entities and use the Navigation Properties:
i.e.
Product p = new Product{
ID = 5,
Name = "Bovril",
Category = ctx.Categories.First( c => c.ID == 5)
};
ctx.AddToProducts(p);
ctx.SaveChanges();
If you want to avoid the database query the easiest approach is probably to use a STUB entity i.e.
// this is a stub, a placeholder for the real entity
Category c = new Category {ID = 5};
// attach the stub to the context, similar to do a query
// but without talking to the DB
ctx.AttachTo("Categories", c);
Product p = new Product{
ID = 5,
Name = "Bovril",
Category = c
};
ctx.AddToProducts(p);
ctx.SaveChanges();
If you want more help on this stub technique check out this blog post on the topic.
Having some experience with Linq to SQL I'm trying out ADO Entity framework now. In Linq to SQL, I would create a Linq to SQL class, drag my tables across to build the data context. Then I'd instantiate the datacontext class and run some lambda against one of the properties in the datacontext class.
Now, with ADO entity framework I add the Entity Data Model class, and add the tables to the data model. My Entity Data Model class now has a bunch of ObjectQuery<> properties, one for each table I added.
Now what do I do with those properties? How do I call them? Anyone have code examples?
Sure. I have a long presentation on this.
As a simple answer to your question, here are some things you can do with the ObjectQuery<T> properties.
Return a list of objects:
IEnumerable<Customer> result = Context.Customers;
return result;
Return one object:
return Context.Customers.Where(c => c.Id == someId).First();
Project onto a presentation model:
return (from c in Customers
where c.Id == someId
select new CustomerPresentation
{
Id = c.Id,
Name = c.Name,
OrderCount = c.Orders.Count(),
PhoneNumbers = from p in c.PhoneNumbers
select new PhoneNumberPresentation
{
AreaCode = p.AreaCode,
// etc.
},
// etc.
}).First();