I am writing some very generic database code, and I would like to find a way to query a set of types in my entity set (using LINQ to SQL EntityFramework, database-first) which are related to another entity.
So, for example I might have a Product belong to a Category;
SELECT * FROM Products p INNER JOIN Categories c ON p.CategoryId = c.Id
In my database, p.CategoryId is a Foreign Key constrain to be NON-NULL. (i.e; a Product MUST belong to a Category).
Now, if I try to DELETE FROM Categories, I get errors, as there are Products still related to Categories, and I do not have (or want) a CASCADE DELETE applied to the relationship.
What I want to do is examine my DataContext library, and determine dynamically that Categories are key'd by Products, and so if I want to DELETE from Categories, I will first need to DELETE from Products.
By 'dynamically', I mean that my code will not have fore-knowledge of the types or relationships (it does not know about Product or Category entities specifically) but I am willing to use Reflection to assess the DLL to work out these relationships.
I have looked at the DataContext-related code, and can see EntityRef properties on certain Entities (which is half the problem solved), but I can't seem to find any way to tell if these relationships are mandatory or optional (i.e; NON-NULL or NULLABLE) in the underlying database.
?
You know you can automatically cascade deletes in Entity Framework by setting the Delete Rule to "Cascade"? That may make things easier (while not necessarily providing a generic solution for LINQ to SQL).
Take a look here for more info:
http://msdn.microsoft.com/en-us/library/bb738695.aspx
Related
I am developping an application using EF6 with Fluent API and I have an issue to manage Many-To-Many relationship.
For some internal reasons the Join table has a specific format including 4 fields
- Left Id (FK)
- Right Id (FK)
- StartDate (dateTime)
- EndDate (datetime)
Deleting a link is in fact setting the EndDate as not null but i don't now how to configure it in EF6.
In an other hand when reading links the record with Not NULL EndDate shouldn't be considered.
Can you give me a solution ?
Thank you.
Join tables and EF
EF automates some things for you. For this, it uses convention-over-configuration. If you stick to the convention, you can skip on a whole lot of common configuration.
For example, if your entity has a property named Id, EF will inherently assume that this is the PK.
Similarly, if two entity types have nav props that refer to each other (and only one direct link between the two entities exists), then EF will automatically assume that these nav props are the two sides to a single many-to-many relationship. EF will make a join table in the database, but it will keep this hidden from you, and let you deal with the two entity types themselves.
For some internal reasons the Join table has a specific format including 4 fields - Left Id (FK) - Right Id (FK) - StartDate (dateTime) - EndDate (datetime)
Your join table no longer conforms to what the content of a conventional and automatically generated EF join table is. You are expecting a level of custom configurability that EF cannot provide based on blind convention, which means you have to explicitly configure this.
Secondly, the fact that you have these additional columns implies that you wish to use this data at some point (presumably to show the historical relations between two entities. Therefore, it doesn't make sense to rely on EF's automatic join tables as the join table and it content would be hidden from the application/developer.
It's possible that the second consideration is invalid for you, if you don't need the application to ever fetch the ended entries. But the overall point still stands.
The solution here is to make the join record an explicit entity of its own. In essence, you are not dealing with a many-to-many here, you are dealing with a specific entity (the join element) with two one-to-many relationships (one for each of the two entity types).
This enables you to achieve exactly what you want. Your expectation of what EF can automate for you simply doesn't apply in this case.
Soft delete
Deleting a link is in fact setting the EndDate as not null but i don't now how to configure it in EF6.
In general, this is known as "soft delete" behavior, albeit maybe slightly differently here. In a regular soft delete pattern, when an entry is deleted, the database secretly retains the entry but the application doesn't know that and doesn't see the entry again.
It's unclear if you intend for ended entries to still show up in the application, e.g. the relational history. If this is not the case, then your situation is exactly soft delete behavior.
This isn't something you configure on the model level, but rather something you override in your database's SaveChanges behavior. A simple example of how I implement a soft delete:
public override int SaveChanges()
{
// Get all entries of the change trackes (of a given type)
var entries = ChangeTracker.Entries<IAuditedEntity>().ToList();
// Filter the entries that are being deleted
foreach (var entry in entries.Where(entry.State == EntityState.Deleted))
{
// Change the entry so it instead updates the entry and does not delete it
entry.Entity.DeletedOn = DateTime.Now;
entry.State = EntityState.Modified;
}
return base.SaveChanges();
}
This allows you to prevent deletions to the entities that you want this to apply to, which is the safest way to implement a soft delete as this serves as a catch-all for database deletes coming from whichever consumer uses this db context.
The solution to your question is pretty much the same. Assuming you named your join entity (see previous chapter) JoinEntity:
public override int SaveChanges()
{
var entries = ChangeTracker.Entries<JoinEntity>().ToList();
// Filter the entries that are being deleted
foreach (var entry in entries.Where(entry.State == EntityState.Deleted))
{
// Change the entry so it instead updates the entry and does not delete it
entry.Entity.Ended = DateTime.Now;
entry.State = EntityState.Modified;
}
return base.SaveChanges();
}
Word of warning
Soft deletes tend to be a catch-all for all entities (or at least a significant chuck of your database). Therefore, it makes sense to catch this at the db context level as I did here.
However, if this entity is unique in that it is soft deleted, then this is more of a business logic implementation than it is a DAL-architecture. If you start writing many custom rules for different types of entities, the db context logic is going to get clutterend and it's not going to be nice to work with because you need to account for multiple possible operations happening during the SaveChanges.
Take note to not push what is supposed to be a business logic decision to the DAL. I can't draw this line for you, it depends on your context. But evaluate whether the db context is the best place to implement this behavior.
Can you give me a solution ?
If your linking table has extra columns you have to model it as an Entity, and the EndDate logic for navigation needs to be explicit. EF won't do any of that for you.
I am using Entity Framework 6.1.3 to generated the entities and data model.
If I have two tables: Orders -> OrderDetails, with a relation between them (OrderId), then I can get all the orders and related OrderDetails with the following query
dbContext.Order().Include(a => a.OrderDetails);
But if I created a view (vOrder) for Orders, then there is no direct relation between vOrder and OrderDetails in the model, though I can link them together with joins on OrderId. How could I still get all the data from vOrder and related OrderDetails. The following query doesn't work unless I add all the navigation properties manually.
dbContext.vOrder().Include(a => a.OrderDetails);
Is there a simple LINQ query to accomplish the intended query?
Thanks for your help.
Do a manual join and return an anonymous object that contains both.
Something like:
dbContext.vOrder
.GroupJoin(
dbContext.OrderDetails,
v=>v.orderid,
od=>o.orderid,
(v,od)=>new {v=v,od=od});
Of course, you could just add the appropriate naviation properties on to vOrder and do exactly what you said.
Why not just include more columns in the view (or create another view that has all the required data, if you don't want to modify the first one)?
In EF cross reference tables are abstracted away by creating many-to-many relationships. E.G.
There's a SQL table dbo.TrialContactCrossReference that relates TrialContactId to TrialID. Now, EF did not generate an Entity TrialContactCrossReference because it went with this MANY-MANY relationship thing. How do I add a new row to said table?
I tried
context.TrialContacts.??? and context.ClinicalTrials.??? and just don't know what to do with this. If I have a new Contact that I want to relate to a trial how am I supposed to go about it?
trial.Contacts.Add(contact);
OR
contact.Trials.Add(trial);
OR (and my advise)
you could create an additional entity for cross reference table. this will convert many-many to 2 one-many relationships. more then %90 cases crosstables has additional columns (at least IsActive, RecordDate etc.) even it doesnt, it may be so in future and it requires you make lots of changes in code.
If I have a new Contact that I want to relate to a trial how am I supposed to go about it?
Assuming you have an existing Contact instance just do:
trial.TrialContacts.Add(contact);
context.SaveChanges();
EF will take care of the intermediate table insert for you. Note that adding Contacts and Trials works the same as if they weren't related.
There's a trick that was not obvious to me when setting this up. TableA must be added to TableB, not just to itself. In fact, looking at the generated entities each entity has a List<> of the other entity.
class TableA
{
List<TableB> TableB;
}
class TableB
{
List<TableA> TableA;
}
For example, if I want to add a TrialContact to a ClinicalTrial then I write:
context.ClinicalTrials.TrialContacts.Add(trialContact);
context.SaveChanges()
Then the xRef table be updated to reflect the relationship.
I have couple of questions with update functionaliy using NHibernate
I have Customer and location entities with 1:n relationship. Customer has location property. While creating/updating customer entity, I just assigned location property and commited changes.
new Location() { Id = ViewModel.LocationId };
Is it proper way to do it or do I need to retrieve the location entity from db and attach it again like below
newCust.Location = GetlocationfromDB(ViewModel.LocationId);
And how does it work with m:n relationship. I have order and orderitems entities. So, if a newgroup is added/deleted, do I need to check which group is added and get from db and attach it or just groupid will do fine..
This isn't the right way to do it - it might work if you have your unsaved-value mapping right for the primary key, but the proper way to do it is to use session.Load(ViewModel.LocationId) see http://ayende.com/blog/3988/nhibernate-the-difference-between-get-load-and-querying-by-id
There are a number of ways of dealing with this, but it sounds like you want your relationship to be mapped as a set (to prevent duplicates) rather than a bag. If you map it as a set and use ISet for the property type of the relationship, the duplicates will be handled for you. If however you use a bag, you would need to remove duplicates in your own code. Again, you should be using session.Load to get the group if it's an already existing group.
I have an entity that maps to a table called Rule. The table for this entity has an FK to another Table called Category. I'm trying to figure out how to pull in a property from Category in my Rule entity. I'm pretty sure I want to use a join in my entity mapping, but I can't figure out how to configure it so that it works. Here is my mapping:
Join("Category", x =>
{
x.Map(i => i.CategoryName, "Name");
x.KeyColumn("CategoryId");
x.Inverse();
});
Here is the SQL that it's generating...
SELECT ...
FROM Rule rules0_ left outer join Category rules0_1_ on rules0_.Id=rules0_1_.CategoryId
WHERE ...
Here is the SQL that I want.
SELECT ...
FROM Rule rules0_ left outer join Category rules0_1_ on rules0_.CategoryId=rules0_1_.Id
WHERE ...
I can't seem to find anything on the JoinPart that will let me do this. Subselect looks promising from the little bit of documentation I've found, but I can't find any examples of how to use it. Any advice on this problem would be much appreciated. Thanks!
"Join" is poorly named. a "join" in an NHibernate mapping implies a zero-to-one relationship based on a relation of the primary keys of the two tables. You would use a join if, for instance, you had a User table and a UserAdditionalInfo table, with zero or one record per User. The UserAdditionalInfo table would likely reference the PK from User as both a foreign key and its own primary key. This type of thing is common when a DBA has to religiously maintain a schema for a legacy app, but a newer app needs new fields for the same conceptual record.
What you actually need in your situation is a References relationship, where a record has a foreign key relationship to zero or one other records. You'd set it up fluently like so:
References(x=>Category)
.Column("CategoryId")
.Inverse()
.Cascade.None();
The problem with this is that Category must now be mapped; it is a separate entity which is now related to yours. Your options are to live with this model, to "flatten" it by making the entity reference private, changing the mapping to access the entity as such, and coding "pass-throughs" to the properties you want public, or by using a code tool like AutoMapper to project this deep domain model into a flat DTO at runtime for general use. They all have pros and cons.