In some scenarios I need a "wide" version of an entity with many properties (say FullCustomer) while in other scenarios it's enough with a "narrow" version with few properties (say MiniCustomer), how could I map these two entities, FullCustomer and MiniCustomer, to the same Customer table in the database? Furthermore, I need to be able to query and update both entities.
Thanks in advance for any advice or pointers!
PD. I'm using VS2010 RC and EF 4
For "efficiency", don't have two versions of the entity; just project onto "lite" POCOs:
var q = from e in Context.Entities
select new LitePoco
{
Id = e.Id,
EditThis = e.EditThis
};
No other columns will be returned.
Similarly for save:
var e = new MyEntity { Id = 123 };
Context.AttachTo("Entities", e);
// anything from here on gets saved
e.EditThis = "Edited";
Context.SaveChanges();
If you have a column that determines whether to treat the record as a FullCustomer or as MiniCustomer, then you can easily create a Table per Hierarchy inheritance model with FullCustomer inherited from MiniCustomer. Such approach is described in the Muhammad Mosa's blog post.
If you don't have this column you can update the model and database in order to add it.
However, there is an alternative approach. If the reason to have two entities for one table is the delay in the loading the numerous large properties in the FullCustomer object, than Table Splitting might be an option. Take a look at the Julie Lerman's blog post.
I would have noted that this problem is not present in ORMs like LINQ to SQL, due to deferred loading.
Related
I am using Entity Framework database-first model. I have 2 tables that are referenced by a link table.
For example:
When I update my edmx file from my database this creates the expected model:
Now what I want to do is delete an entry from the Product_User table without deleting the referenced entry in either of the related tables (Product or User).
I've tried both of these statements (together and separately) but neither seems to have any effect:
user.Products.Clear();
foreach (var product in products)
{
product.User = null;
}
Is what I'm trying to do possible with the model the way I have it now? And if so what am I doing wrong?
I noticed I can do what I'm trying to do if I add the link table explicitly to the model but I'm trying to avoid that.
Any help would be much appreciated. Thanks.
Your seconde way to go (with a loop) seems more like the correct way.
Instead of = null try to use the .Remove() function. I guess the remove function only remove the link between the two entity, not the entities related.
See this answer : Removing many to many entity Framework
I am new to EF so please bear with me. I am using Entity Framework with an existing database in which the relationships are not defined.
Using EF, I am able to produce the model, but obviously the 'navigational properties' are not working. Is there a way I can specify the mapping between the entities?
For example, in my Product entity, I have a field CategoryID_fk (maps to Category entity). But since the relationships are not defined, I cannot load a Category while loading a Product entity.
Can someone guide me in this regard?
I do understand that it would be preferable to refactor our database but I am unable to do that now. Thanks in advance.
This link is very useful.http://www.entityframeworktutorial.net/
I think field names are not suitable conventions.Try to change field name as CategoryId or Category_CategoryId .I think it will work
Yes, you can do a group join. Basically you will retrieve Products and Category in separate queries and take an approach as indicated in this answer.
Something along these lines:
var productCategoryList = products.GroupJoin
(
categories,
p=> p.productId,
c=> c.CategoryId,
(p, c) => new ProductCategory
{
//Create your Product Category model here
}
).AsEnumerable();
I have a legacy database with a particular table -- I will call it ItemTable -- that can have billions of rows of data. To overcome database restrictions, we have decided to split the table into "silos" whenever the number of rows reaches 100,000,000. So, ItemTable will exist, then a procedure will run in the middle of the night to check the number of rows. If numberOfRows is > 100,000,000 then silo1_ItemTable will be created. Any Items added to the database from now on will be added to silo1_ItemTable (until it grows to big, then silo2_ItemTable will exist...)
ItemTable and silo1_ItemTable can be mapped to the same Item entity because the table structures are identical, but I am not sure how to set this mapping up at runtime, or how to specify the table name for my queries. All inserts should be added to the latest siloX_ItemTable, and all Reads should be from a specified siloX_ItemTable.
I have a separate siloTracker table that will give me the table name to insert/read the data from, but I am not sure how I can use this with entity framework...
Thoughts?
You could try to use the Entity Inheritance to get this. So you have a base class which has all the fields mapped to ItemTable and then you have descendant classes that inherit from ItemTable entity and is mapped to the silo tables in the db. Every time you create a new silo you create a new entity mapped to that silo table.
[Table("ItemTable")]
public class Item
{
//All the fields in the table goes here
}
[Table("silo1_ItemTable")]
public class Silo1Item : Item
{
}
[Table("silo2_ItemTable")]
public class Silo2Item : Item
{
}
You can find more information on this here
Other option is to create a view that creates a union of all those table and map your entity to that view.
As mentioned in my comment, to solve this problem I am using the SQLQuery method that is exposed by DBSet. Since all my item tables have the exact same schema, I can use the SQLQuery to define my own query and I can pass in the name of the table to the query. Tested on my system and it is working well.
See this link for an explanation of running raw queries with entity framework:
EF raw query documentation
If anyone has a better way to solve my question, please leave a comment.
[UPDATE]
I agree that stored procedures are also a great option, but for some reason my management is very resistant to make any changes to our database. It is easier for me (and our customers) to put the sql in code and acknowledge the fact that there is raw sql. At least I can hide it from the other layers rather easily.
[/UPDATE]
Possible solution for this problem may be using context initialization with DbCompiledModel param:
var builder = new DbModelBuilder(DbModelBuilderVersion.V6_0);
builder.Configurations.Add(new EntityTypeConfiguration<EntityName>());
builder.Entity<EntityName>().ToTable("TableNameDefinedInRuntime");
var dynamicContext = new MyDbContext(builder.Build(context.Database.Connection).Compile());
For some reason in EF6 it fails on second table request, but mapping inside context looks correct on the moment of execution.
Background is the team i'm in has just started using the EntityFramework; first we designed the database, put all the table relationships in place, foreign keys, etc; then thru visual studio add a new ADO.NET Entity Data Model, and auto-magically we get the generated edmx file representing the whole database !
Now i focus on two tables that provide data for all dropdowns and lookup lists;
TLookupDomain (domainID, domainName, domainDesc )
TLookup (lookupID, domainID, lookupCode, lookupDisplay, lookupDesc, sortOrder)
Relationship is 1-M going from left to right:
TLookupDomain -< TLookup -< TOther (+ another 30 or so other tables)
So lookupID is a foreign-Key to as many as 30 tables;
IQueryable<TLookup> qList = from l in ctx.TLookups
where l.domainID == 24
select l;
foreach (TLookup l in qList)
{
//do something.
System.Diagnostics.Debug.WriteLine("{0}\t{1}", l.lookupCode, l.lookupDisplay);
foreach (TOther f in l.TOthers)
{
System.Diagnostics.Debug.WriteLine("{0}\t{1}", f.feeAmount, f.feeDesc);
}
}
When i execute the above LINQ, i get all the fields for TLookup table (which is fair), BUT data is also fetched for the 30 or so tables that are linked to it, even though i am NOT interested in the other table's data at this point, and i am going to discard all data soon as LINQ fetches it.
Two Questions i have:
Q.1) Can i somehow modify the LINQ query above or tell the EntityFramework otherwise not to bother fetchin data from the 30 other linked tables ?
Q.2) is it "right" to have one edmx file that models the entire database? (sounds dodgy to me).
Configure Lazy Load to true for the model. Relations should be loaded only upon navegation. You can also split the models to avoid too many unnecessary relations.
Linq-to-Entities queries do not fetch anything automatically. Fetching of navigation properties is performet either by eager or lazy loading. You are not using eager loading because that requires calling Include in query (or ctx.LoadProperty separately). So if your data are fetched it must be due to lazy loading wich is enabled by default. Lazy loading triggers once you access the navigation property in the code.
You can also return only the data you need by using projections. Something like this should return readonly data:
var query = from l in ctx.TLookups
where l.domainId == 24
select new
{
l.lookupCode,
l.lookupDisplay,
l.TOthers
};
Having one or more EDMX is common dilemma. Working with single EDMX makes things more simple. If you want to know how to use multiple EDMXs and share conceptual definitions check these two articles: Part 1, Part 2.
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.