EF6 has an overload of DbSet.Include which accepts a string parameter representing a dot-separated list of related objects to return in the query results. It is useful for eager-loading entities in a multi-level object graph. For example:
var order = await _dbContext.Orders
.Include(o => o.Customer)
.Include("OrderDetails.Product") // dot-delimited path
.SingleOrDefaultAsync(o => o.OrderId == id);
This will return both related order details and populate the Product property of each detail by generating a SQL statement that joins OrderDetail and Product tables.
I am looking a way to do this with EF7, but I don't see an overload of DbSet.Include which accepts a string path parameter. Does EF7 provide a way to achieve the same result as the EF6 API?
PS. I just noticed issue #1151 is open, and it looks like it may address my question.
You are correct that #1151 is tracking this scenario. There are also some design meeting notes that summarize the API that will be available in EF7 - https://github.com/aspnet/EntityFramework/wiki/Design-Meeting-Notes:-January-8,-2015.
Related
i would like to know if you have any idea how i can achieve this, considering a query stored as string in the configuration file.
I tried to use SqlQuery applied to the DBSet, but the problem is that SqlQuery requires me to select all properties of the required entities in my query. If i don't consider any column, it will complain because is not able to map the query to the entities.
I don't want to select all properties of the entities i want to query.
Thanks
If you are using EF then why not use Database.ExecuteSqlCommand()? It's in the System.Data.Entity namespace.
For example:
int result = db.Database.ExecuteSqlCommand("Non SELECT SQL etc...");
Well, I ended up implementing a mechanism using reflection that basically receives a group of fields to select, and constructs dynamic objects with those fields, so when applied the query with the joins between the entities, will only bring the fields I am looking for.
So, considering Entity1, Entity2, Entity3 with the following relationship
<b>Entity1</b>{
<br/> Entity1Name, <br/> List<*Entity2*> Entity2Items, <br/> etc..
<br/>}
and
<b>Entity2</b> { <br/> Entity2Name, <br/> List<*Entity3*> Entity3Items <br/>}
I can store e.g. the following query in the configuration file, and retrieve the information:
"Entity1.Entity1Name", <br/>
"Entity1.Entity2Items.Entity2Name", <br/>
"Entity1.Entity2Items.Entity3Items.Entity3Name"
Anyway, I was just trying to see if there would be any solution out-of-the-box that would require minimal code changes.
Thank you.
If I have this
var selectedEntities = db.MyEntities.Include(item => item.RelatedEntities);
It will load all properties (columns) in MyEntities but also all properties in ReleatedEntities. If I only need one property from RelatedEntities, how would I specify that?
use .Select() and anonymous type to restrict the columns you want
var result = await _appDbContext.Companies
.AsNoTracking()
.Select(company => new
{
Company = company,
EmployeeIds = company.Employees.Select(emp => emp.Id)
})
.ToListAsync()
.ConfigureAwait(false);
I was looking for the same, and after referring to other questions it seems like it's not possible. The understanding i got is that an EF entity is represented by the collection of it's properties, and hence loading individual values will not fully define the Entity.
To load only selected properties, we need to use Select query, instead of loading the related data. The query will result in an anonymous type.
Note : If the resulting type contains any entity, then the changes will be tracked. Refer Tracking and projections
References :
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/basic-linq-query-operations#selecting-projections
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/projection-operations
How to include only selected properties on related entities
https://learn.microsoft.com/en-us/ef/core/querying/tracking#tracking-and-projections
(as advised re-posting this question here... originally posted in msdn forum)
I am striving to write a "generic" routine for some simple CRUD operations using EF/Linq to Entities. I'm working in ASP.NET (C# or VB).
I have looked at:
Getting a reference to a dynamically selected table with "GetObjectByKey" (But I don't want anything from cache. I want data from database. Seems like not what this function is intended for).
CRM Dynamic Entities (here you can pass a tablename string to query) looked like the approach I am looking for but I don't get the idea that this CRM effort is necessarily staying current (?) and/or has much assurance for the future??
I looked at various ways of drilling thru Namespaces/Objects to get to where I could pass a TableName parameter into the oft used query syntax var query = (from c in context.C_Contacts select c); (for example) where somehow I could swap out the "C_Contacts" TEntity depending on which table I want to work with. But not finding a way to do this ??
Slightly over-simplyfing, I just want to be able to pass a tablename parameter and in some cases some associated fieldnames and values (perhaps in a generic object?) to my routine and then let that routine dynamically plug into LINQ to Entity data context/model and do some standard "select all" operations for parameter table or do a delete to parameter table based on a generic record id. I'm trying to avoid calling the various different automatically generated L2E methods based on tablename etc...instead just trying to drill into the data context and ultimately the L2E query syntax for dynamically passed table/field names.
Has anyone found any successful/efficient approaches for doing this? Any ideas, links, examples?
The DbContext object has a generic Set() method. This will give you
from c in context.Set<Contact>() select c
Here's method when starting from a string:
public void Test()
{
dynamic entity = null;
Type type = Type.GetType("Contract");
entity = Activator.CreateInstance(type);
ProcessType(entity);
}
public void ProcessType<TEntity>(TEntity instance)
where TEntity : class
{
var result =
from item in this.Set<TEntity>()
select item;
//do stuff with the result
//passing back to the caller can get more complicated
//but passing it on will be fine ...
}
I am just starting to use the Entity Framework 4 for the first time ever. So far I am liking it but I am a bit confused on how to correctly do inheritance.
I am doing a model-first approach, and I have my Person entity with two subtype entities, Employee and Client. EF is correctly using the table per type approach, however I can't seem to figure out how to determine what type of a Person a specific object is.
For example, if I do something like
var people = from p in entities.Person select p;
return people.ToList<Person>();
In my list that I form from this, all I care about is the Id field so i don't want to actually query all the subtype tables (this is a webpage list with links, so all I need is the name and the Id, all in the Persons table).
However, I want to form different lists using this one query, one for each type of person (so one list for Clients and another for Employees).
The issue is if I have a Person entity, I can't see any way to determine if that entity is a Client or an Employee without querying the Client or Employee tables directly. How can I easily determine the subtype of an entity without performing a bunch of additional database queries?
Use .OfType<Client>() in your query to get just the clients. See OfType.
e.g. entities.Person.OfType<Client>() ...
Use is to test if a Person object is a specific sub-class, e.g. if (p is Employee) ...
BTW why isn't it entities.People? Did you not select the pluralization option?
Greetings,
Considering the Northwind sample tables Customers, Orders, and OrderDetails I would like to eager load the related entities corresponding to the tables mentioned above and yet I need ot order the child entities on the database before fetching entities.
Basic case:
var someQueryable = from customer in northwindContext.Customers.Include("Orders.OrderDetails")
select customer;
but I also need to sort Orders and OrderDetails on the database side (before fetching those entities into memory) with respect to some random column on those tables. Is it possible without some projection, like it is in T-SQL? It doesn't matter whether the solution uses e-SQL or LINQ to Entities. I searched the web but I wasn't satisfied with the answers I found since they mainly involve projecting data to some anonymous type and then re-query that anonymous type to get the child entities in the order you like. Also using CreateSourceQuery() doesn't seem to be an option for me since I need to get the data as it is on the database side, with eager loading but just by ordering child entities. That is I want to do the "ORDER BY" before executing any query and then fetch the entities in the order I'd like. Thanks in advance for any guidance. As a personal note, please excuse the direct language since I am kinda pissed at Microsoft for releasing the EF in such an immature shape even compared to Linq to SQL (which they seem to be getting away slowly). I hope this EF thingie will get much better and without significant bugs in the release version of .NET FX 4.0.
Actually I have Tip that addresses exactly this issue.
Sorting of related entities is not 'supported', but using the projection approach Craig shows AND relying on something called 'Relationship Fixup' you can get something very similar working:
If you do this:
var projection = from c in ctx.Customers
select new {
Customer = c,
Orders = c.Orders.OrderByDescending(
o => o.OrderDate
)
};
foreach(var anon in projection )
{
anon.Orders //is sorted (because of the projection)
anon.Customer.Orders // is sorted too! because of relationship fixup
}
Which means if you do this:
var customers = projection.AsEnumerable().Select(x => x.Customer);
you will have customers that have sorted orders!
See the tip for more info.
Hope this helps
Alex
You are confusing two different problems. The first is how to materialize entities in the database, the second is how to retrieve an ordered list. The EntityCollection type is not an ordered list. In your example, customer.Orders is an EntityCollection.
On the other hand, if you want to get a list in a particular order, you can certainly do that; it just can't be in a property of type EntityCollection. For example:
from c in northwindContext.Customers
orderby c.SomeField
select new {
Name = c.Name,
Orders = from o in c.Orders
orderby c.SomeField
select new {
SomeField = c.SomeField
}
}
Note that there is no call to Include. Because I am projecting, it is unnecessary.
The Entity Framework may not work in the way you expect, coming from a LINQ to SQL background, but it does work. Be careful about condemning it before you understand it; deciding that it doesn't work will prevent you from learning how it does work.
Thank you both. I understand that I can use projection to achieve what I wanted but I thought there might be an easy way to do it since in T-SQL world it's perfectly possible with a few nested queries (or joins) and order bys. On the other hand seperation of concerns sounds reasonable and we are in the entity domain now so I will use the way you two both recommended though I have to admit this is easier and cleaner to achieve in LINQ to SQL by using AssociateWith.
Kind regards.