I have the following database situation:
wp_users (user table generated by wordpress)
ID | user_login | ...
wp_sp_user (extension to the wp_users table)
ID (FK) | surname | address | ...
Now I've already been trying for hours to "fuse" those two tables into one single User entity, e.g:
class User {
var ID;
var user_login;
var surname;
var address;
...
}
Is there any way to accomplish such a mapping without modifying the wp_user table (which I don't want to do for updating reasons)?
Some times database refactoring is not possible or the table has his own "raison d'être". In this cases you can use inheritance. Your User class can extens Account. Map Account to wp_users and extend it with wp_sp_user table. User class will use columns of the two tables.
Here is the doctrine documentation:
https://www.doctrine-project.org/projects/doctrine-orm/en/current/reference/inheritance-mapping.html
This is not possible. It also doesn't make sense to do so.
You will need to physically merge the tables together in MySQL and create a Doctrine entity for that table. This is the only way you can ensure your data is clean and fully normalized.
Another possible solution is to create one entity for each table and use a business object to combine results from each. This is not a very nice solution at all, as you will have to handle constraints on the application layer, and you will double the amount of queries you launch.
Related
Lets say I have a parent/child relationship models with 2 google datastore kinds.
If I query the child table using the parent key, is there any way of also retrieving the fields from the parent table without having to do n+1 queries ?
RequestLedger
key | type | content | sentTimestamp
123 email <ssd> 10-10-10
ResultLedger
key | requestLedger |to | deliveredTimestamp | bouncedTimestamp | other
123-xxx#xxx.com key(request_ledger,123) xxx#xxx.com 10-10-10
code
var query = ds.createQuery(env.get('GCLOUD_DATASTORE_NAMESPACE'), resultLedgerKind)
.offset(offset)
.limit(max);
if(parentId){
query = query.filter('requestLedger', ds.key([requestLedgerKind, parentId]));
}
query.run(function(err, entities) {
callback(err, entities );
});
Query above gives me data from the child table and a reference to the parent kind entity but only the key field of the parent. Any easy eay to get everything back in the query at same time?
You can use the lookup method to get multiple entities by their keys in one request. The rest example is here I'm convinced the javascript implementation has a similar implementation.
Note that since api v1 there is a limit of 1000 keys you can get in a single request. This wasn't so in the beta version.
This should bring back your request count if you first get the children and then lookup their respective parent keys.
I have the following tables created using SQL Server management studio (I must not let EF create the DB because I need customized indexing along with something else).
Table A (AId)
Table B (BId)
Table AB(AId, BId)
There are foreign key constrains setup so A - B is a n-n relationship.
Now, when I import the model into EF, the relationship is displayed as * - * (which is correct), but Table AB is gone. This is not what I want! I want to have the ability to manually manage the relationship. How can I have EF show this table?
(the reason is the way EF manage relationship through strongly typed objects is counter performance - e.g. I want to manually create a B and link it to some As (I know the ID) without selecting those As and adding it in B's collection)
As Slauma stated it is possible to add a dummy field, but more importantly you can remove that dummy field and still have the link table.
Simple create the table AB, with an extra column called "Dummy" for example. Generate your model from this table. The table will now be included in the model.
Now got to the database and drop the "Dummy" column.
You can now either refresh the model from the database of simple delete the mapping for the "Dummy" column. You now have the link table in your EF model.
It's not possible to force EF to expose the link table as an entity when you create the model via database first - unless you change the schema in a way that EF does not consider the table as a pure many-to-many link table anymore, for example by adding a "dummy column" to the table. When you are using Code-First however, you can manually create an entity for the link table with two one-to-many relationships between A and AB and between B and AB.
I am not convinced by your argument why you want to do that...
I want to manually create a B and link it to some As (I know the ID)
without selecting those As and adding it in B's collection.
...because you don't need to load any entities from the database if you know the key values. You can use attached stub entities in order to create the relationships and write entries into the link table:
var newB = new B { As = new List<A>() };
foreach (var aId in someAIdCollection)
{
var existingA = new A { Id = aId };
context.As.Attach(existingA);
newB.As.Add(existingA);
}
context.Bs.Add(newB);
context.SaveChanges();
No database query is involved here and the resulting SQL commands are the same like inserting an AB link entity directly.
I use Entity Framwork 4.1 and MVC 3. My question is about Join tables in Entity Framework.
I created three tables
User - UserId, Username
Role - Role Id, Rolename
UserInRoles- UserId, RoleId
I pulled these tables to my edmx file,
Only two tables appear i.e. the User and Role with a Many to Many Association between two.
If I want to get a user's role from UserInRoles table, What is the approach?
I did something like
var result = (from ar in roles
from au in users
where au.UserName == username
select new {});
This is not working. Please help.
As you guessed, the Entity Framework does combine simple look up tables like that into the parent tables in an effort to help you out. You won't be able to access the look up table in your code, but it will do the heavy lifting and allow you to navigate through the tables like such.
var result = (from ar in roles
where ar.Users.UserName == username
select ar);
Or if you prefer lambdas (personal preference):
var result = context.roles.Where(x => x.Users.UserName == username);
They should be linked I suppose:
var result =
from au in users
where au.UserName == username
select au.Roles;
Not sure if it will give you an empty collection by default. The Roles collection may have to be manually fetched. (Possibly result.Fetch(), but I'm not too familiar with it.)
(Or do you wat access to the actual UserInRole items that are stored in the database?)
Entity framework automatically generates a many to many relationship in the model, when there is only a relationshiptable with one shared primarykey that are foreign keys to other tables.
You can either add another column to this table to add it to your model or generate another unique key for the relationship.
Read more about this here :
http://learnentityframework.com/LearnEntityFramework/tutorials/many-to-many-relationships-in-the-entity-data-model/
Using Entity Framework, I am writing a social networking app and trying to setup some relationships so that I can have Users with Followers/Following properties.
I have a Users table:
Users
_______
Id
FirstName
LastName
Email
Then I have a Follows table:
Follows
__________
FollowerId
FolloweeId
On a strongly typed User object, I want to see .Followers and .Following properties that return collections of User objects. So far I've tried making the columns in the Follows table as foreign keys, and both as composite primary keys. The entity model comes out the way I want it (I don't have an Entity in-between, and I just have to rename the navigation properties), but when I populate with data, the Followers and Following collections are empty, so something is not right with the relationships.
I suppose I could separate and have two tables, Followers and Following, but I would have duplicate data and would have to add to two tables when someone follows someone else.
You need to include the data for the tables you User tables has relationships with in your query.
A query like this:
using (EntityObject context = new EntityObject())
{
var user = from x in context.users.Include("Followers").Include("Followees") //use your tables
where x.Id == TheUserId
select x;
}
Will let you have access to the objects for those included tables.
i have two tables
asset employee
assetid-pk empid-pk
empid-fk
now, i have a form to populate the asset table but it cant because of the foreign key constraint..
what to do?
thx
Tk
Foreign keys are created for a good reason - to prevent orphan rows at a minimum. Create the corresponding parent and then use the appropriate value as the foreign key value on the child table.
You should think about this update as a series of SQL statements, not just one statement. You'll process the statements in order of dependency, see example.
Asset
PK AssetID
AssetName
FK EmployeeID
etc...
Employee
PK EmployeeID
EmployeeName
etc...
If you want to "add" a new asset, you'll first need to know which employee it will be assigned to. If it will be assigned to a new employee, you'll need to add them first.
Here is an example of adding a asset named 'BOOK' for a new employee named 'Zach'.
DECLARE #EmployeeFK AS INT;
INSERT (EmployeeName) VALUES ('Zach') INTO EMPLOYEE;
SELECT #EmployeeFK = ##IDENTITY;
INSERT (AssetName, EmployeeID) VALUES ('BOOK',#EmployeeFK) INTO ASSET;
The important thing to notice above, is that we grab the new identity (aka: EmployeeID) assigned to 'Zach', so we can use it when we add the new asset.
If I understand you correctly, are you trying to build the data graph locally before persisting to the data? That is, create the parent and child records within the application and persist it all at once?
There are a couple approaches to this. One approach people take is to use GUIDs as the unique identifiers for the data. That way you don't need to get the next ID from the database, you can just create the graph locally and persist the whole thing. There's been a debate on this approach between software and database for a long time, because while it makes a lot of sense in many ways (hit the database less often, maintain relationships before persisting, uniquely identify data across systems) it turns out to be a significant resource hit on the database.
Another approach is to use an ORM that will handle the persistence mapping for you. Something like NHibernate, for example. You would create your parent object and the child objects would just be properties on that. They wouldn't have any concept of foreign keys and IDs and such, they'd just be objects in code related by being set as properties on each other (such as a "blog post" object with a generic collection of "comment" objects, etc.). This graph would be handed off to the ORM which would use its knowledge of the mapping between the objects and the persistence to send it off to the database in the correct order, perhaps giving back the same object but with ID numbers populated.
Or is this not what you're asking? It's a little unclear, to be honest.