I am trying to implement a custom task scheduler system.
I have a following (simplified) entity model:
class User
{
public virtual long UserId { get; set; }
public virtual string Name { get; set; }
}
class Task
{
public virtual long TaskId { get; set; }
public virtual long? UserId { get; set; }
public virtual string TaskName { get; set; }
public virtual Guid SchedulerSessionUid { get; set; }
}
For now, corresponding SQL tables are straight forward with fields mapping exactly as they appear in the classes above.
The scheduler is a C# Windows console app. It will run once per day.
It should work in a following way (simplified):
generate a Guid for current session
try selecting next User entity, which does not already have a task scheduled in the current scheduler session (Guid compare); if no such a User found, go to the step 5.
add a new Task for the user selected in the step 2.
go to step 2.
exit the application
It seems a pretty trivial problem, but I have a problem implementing the second step. I have tried various queries, but there are some rules which always stop me.
Here are some rules which I have to obey while implementing the step 1:
I am not allowed to modify the User class or User SQL table
I may modify Task class and table, if it will help to solve the problem
I may add a new table or a stored procedure, if it will help
I have to implement it in a way which is as compatible with NHibernate and LINQ as possible
there might exist also some tasks which are not associated with any User object (the scheduler will ignore those, but still I have to keep that in mind while designing the SQL/LINQ query and NHibernate mapping).
Here are some real world example how it should work.
Users
-----------------
UserId Name
-----------------
1 First
2 Second
Tasks
--------------------------------------------------------
TaskId UserId SchedulerSessionUid
--------------------------------------------------------
1 NULL 6d8e48d0-4e92-477e-82fa-cd957e7dc201
2 1 d213cfc8-23d6-49fb-b4e3-9ff3b60af6c4
3 1 9ee042df-88a7-447e-adbd-e7551ed50ae5
1.Now when the Scheduler runs, it generates a current session id = 76ea57fa-8c89-4c05-9ca2-a450b1f8a032.
Now it should issue the magical LINQ query to NHibernate LINQ
provider to get a User entity
In the first iteration the query should return the User entity with
UserId=1 because there are no tasks in the current session for that
User yet
Now the Scheduler creates a new task with UserId=1,
SchedulerSessionUid=76ea57fa-8c89-4c05-9ca2-a450b1f8a032.
In the next iteration the Scheduler should get a User with UserId=2. Again, a new task is inserted with UserId=2, SchedulerSessionUid=76ea57fa-8c89-4c05-9ca2-a450b1f8a03.
In the next iteration the Scheduler should get no users, so it exits.
What LINQ query could I use to get the User for the step 2? What changes in my SQL schema and entity model do I need?
If I now follow you correctly, you need to get a (just one) user for which there are no tasks with the given session id. Am I correct?
Users.Where(u => !Tasks.Any(t = > t.UserId == u.UserId && t.SchedulerSessionUid == curSession)).FirstOrDefault()
Edit:
Since you're doing several spins through this, would you perhaps be faster doing:
foreach(var user toDealWith Users.Where(u => !Tasks.Any(t = > t.UserId == u.UserId && t.SchedulerSessionUid == curSession)))
{
//do stuff
}
Rather than keep hitting the database each time?
Related
I have a simple class to represent a tree structure, defined like this:
public class LicenceCategory
{
[Key]
[Column("LicenceCategoryID")]
public Guid ID { get; set; }
public string Name { get; set; }
public Guid? ParentLicenceCategoryID { get; set; }
[ForeignKey("ParentLicenceCategoryID")]
public virtual List<LicenceCategory> Categories { get; set; }
}
Then, from an ASP.NET Core controller, I simply return myContext.LicenceCategory, which is about as simple as it gets.
Right now, there are five records on the database: the parent (null ParentLicenceCategoryID), and four children for that one parent. So no massive volumes and no very deep nesting. This is the SQL that gets generated, and is as I expect:
SELECT [obj].[LicenceCategoryID], [obj].[Name], [obj].[ParentLicenceCategoryID]
FROM [LicenceCategory] AS [obj]
However, it also generates this, five times:
SELECT [e].[LicenceCategoryID], [e].[Name], [e].[ParentLicenceCategoryID]
FROM [LicenceCategory] AS [e]
WHERE [e].[ParentLicenceCategoryID] = #__get_Item_0
Notice how the first statement already contains every field you need to build the tree structure client-side. Why on earth even do the extra select statements?
I noticed that if I Include navigation properties, things get much worse: For three navigation properties, I wound up with 21 select statements! Most of which are just the same statement executed again and again and again. It may do so with different parameters perhaps, but there is hardly any way to make a program any less efficient. And these are five records - what will EF do when I throw our millions of transaction records its way?
Is there a way to prevent this kind of code generation, or is EF Core simply a non-starter?
I am making an MVC 3 web application in asp.net. It is supposed to be like a little community where users can send messages to each other (and later on also to specified groups of people). So in my database I have the tables Users and Messages. A row in Messages (a message) contains both sender and receiver (a foreign key to the Users table) plus some other info such as title, send date and of course the message text.
When a user come to the index page for logged in users, I want to show all messages that have been sent to this user, or sent from this user. Therefore, when reading the messages from the Messages table I want to use a parameter, the id of the logged in user (which I can access in my controller). So I have the parameter but I don't know how to use it when getting the results. I know there must be a way to do this correctly instead of putting in a bunch of sql and starting connections right in the code the ugly way.
As you maybe can see now, all the messages from the table will be retrieved and sent to the View for displaying it to the user.
The CommunityEntities line is part of the whole Entity Framework thing, not exactly sure how that stuff works yet, although I have been following some tutorials:
(Creating-an-entity-framework-data-model-for-an-asp-net-mvc-application)
(Mvc-music-store-part-1).
UserController.cs:
...
CommunityEntities db = new CommunityEntities();
public ActionResult Index()
{
var messages = db.Messages.ToList();
return View(messages);
}
...
CommunityEntities.cs:
public class CommunityEntities : DbContext
{
public DbSet<User> Users { get; set; }
public DbSet<Message> Messages { get; set; }
}
EDIT: I did try that thing with db.Database.SqlQuery(...) but that seemed wrong, seeing as the point is to do this correctly and "neat". This is for my education and my teachers are going to be picky at the structure of the application.
You need a where clause:
var messages = db.Messages.Where( m => m.SentById == id || m.SentToId == id ).ToList();
Error Message: "Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries."
Hello All,
I've created a Code First TPH(Table Per Hierarchy) within MVC and EF with a SQL compact db.
Here's the Class diagram/Hierarchy:
Class Diagram
Client and SalesRep both inherit the BaseUser class. The Key is "UserID" and it's coded with the Data Annotation [Key](I'm aware that 'ID' should set it as well)
Here's where I'm at: I can seed the database with a few entries. When I try to set the "UserID" in the seeding method it seems to ignore it and just apply the UserID in numerical order...(Seems ok to me?)
furthermore here's my DbContext
public class SiteDB:DbContext
{
public DbSet<BaseUser> AllUsers { get; set; }//enable TPH
public DbSet<SalesRep> SalesReps { get; set; }
public DbSet<Client> Clients { get; set; }
}
Next,
I have created a controller for Clients -> ClientsController with strongly typed Razor Views. With this. I now have a CRUD for the Clients. I can create new Clients without any issue, but when I try to edit a Client entry, I get the error message stated above.
I did notice something interesting I stepped through the code and the error is happening on the db.SaveChanges();
When the client is passed back into the ActionResult Edit method the UserID=0! Bizarre? I'm not sure if this is a bug or if it's an actual issue that's causing this.
UserID=0
Your help with this is appreciated. Thanks!
To modify an entity you need to get it from the database: when you pass back the modified values in the Edit action you need to retrive the entity from the db object, getting it by ID, apply the modified values and save it.
Here an example:
public ActionResult Edit(int id, MyModel model){
using (SiteDBdb = new SiteDB()){
Client cl = (from c in db.Clients where c.id == id select c).First();
cl.MyProp = model.MyProp;
...
db.SaveChanges();
}
...
}
I am working on an NHibernate project and have a question regarding updating transient entities.
Basically the workflow is as follows:
Create a DTO (projection) and send over the wire to client. This has a small subset of properties from the entity.
Client sends back the changed DTO
Map the DTO properties back onto the appropriate enitity so an UPDATE statement can be generated and executed by NH.
Save the entity
Point 4 is where I have the issue. Currently I can achieve this update using the session.Merge() method, however it must first load the entity from the db (assume no 2LC) before updating. So, both a select and an update statement are fired.
What I would like to do is create a transient instance of the entity, map the new values from the DTO, then have NH generate a SQL statement using only the properties I have changed. The additional select should be unnecessary as I already have the entity ID and the values required for the SET clause. Is this possible in NH?
Currently using session.Update(), all properties will be included in the update statement and an exception is raised due to the uninitialized properties that are not part of the DTO.
Essentially I need a way to specify which entity properties are dirty so only these are included in the update.
== EDIT ==
For example...
public class Person
{
public virtual int PersonId { get; set; }
public virtual string Firstname { get; set; }
public virtual string Nickname { get; set; }
public virtual string Surname { get; set; }
public virtual DateTime BirthDate { get; set; }
}
And the test case.
// Create the transient entity
Person p = new Person()
p.id = 1;
using (ISession session = factory.OpenSession())
{
session.Update(p);
// Update the entity – now attached to session
p.Firstname = “Bob”;
session.Flush();
}
I was hoping to generate a SQL statement similar to ‘UPDATE Persons SET Firstname = ‘Bob’ WHERE PersonID = 1’. Instead I get a DateTime out of range exception due to BirthDate not being initialised. It shouldn’t need BirthDate as it is not required for the SQL statement. Maybe this isn’t possible?
== /EDIT ==
Thanks in advance,
John
Dynamic-update is what you're looking for. In your mapping file (hbm.xml):
<class name="Foo" dynamic-update="true">
<!-- remainder of your class map -->
Be aware of the potential problems that this may cause. Let's say you have some domain logic that says either FirstName or Nickname must not be null. (Completely making this up.) Two people update Jon "Jonboy" Jonson at the same time. One removes his FirstName. Because dynamic-update is true, the update statement just nulls out Jon and the record is now "Jonboy" Jonson. The other simultaneous update removes his Nickname. The intent is Jon Jonboy. But only the null-out of the Nickname gets sent to the database. You now have a record with no FirstName or Nickname. If dynamic-update had been false, the second update would have set it to Jon Jonboy. Maybe this isn't an issue in your situation, but setting dynamic-update="true" has consequences and you should think through the implications.
UPDATE: Thanks for the code. That helped. The basic problem is NHibernate not having enough information. When you say session.Update(p), NHibernate has to associated a disconnected entity with the current session. It has a non-default PK. So NHibernate knows that it's an update and not an insert. When you say session.Update(p), NHibernate sees the whole entity as dirty and sends it to the database. (If you use session.Merge(obj), NHibernate selects the entity from the database and merges obj with it.) This is not what you really mean. You want to associate your object with the current session, but mark it as clean. The API is somewhat non-intuitive. You use session.Lock(obj, LockMode.None) as below.
using(var session = sessionFactory.OpenSession())
using(var tx = session.BeginTransaction()) {
var p = new Person {PersonId = 1};
session.Lock(p, LockMode.None); // <-- This is the secret sauce!
p.Firstname = "Bob";
// No need to call session.Update(p) since p is already associated with the session.
tx.Commit();
}
(N.B. dynamic-update="true" is specified in my mapping.)
This results in the following SQL:
UPDATE Person
SET Firstname = 'Bob' /* #p0_0 */
WHERE PersonId = 1 /* #p1_0 */
I have a question that I may be over thinking at this point but here goes...
I have 2 classes Users and Groups. Users and groups have a many to many relationship and I was thinking that the join table group_users I wanted to have an IsAuthorized property (because some groups are private -- users will need authorization).
Would you recommend creating a class for the join table as well as the User and Groups table? Currently my classes look like this.
public class Groups
{
public Groups()
{
members = new List<Person>();
}
...
public virtual IList<Person> members { get; set; }
}
public class User
{
public User()
{
groups = new Groups()
}
...
public virtual IList<Groups> groups{ get; set; }
}
My mapping is like the following in both classes (I'm only showing the one in the users mapping but they are very similar):
HasManyToMany<Groups>(x => x.Groups)
.WithTableName("GroupMembers")
.WithParentKeyColumn("UserID")
.WithChildKeyColumn("GroupID")
.Cascade.SaveUpdate();
Should I write a class for the join table that looks like this?
public class GroupMembers
{
public virtual string GroupID { get; set; }
public virtual string PersonID { get; set; }
public virtual bool WaitingForAccept { get; set; }
}
I would really like to be able to adjust the group membership status and I guess I'm trying to think of the best way to go about this.
I generally only like to create classes that represent actual business entities. In this case I don't think 'groupmembers' represents anything of value in your code. To me the ORM should map the database to your business objects. This means that your classes don't have to exactly mirror the database layout.
Also I suspect that by implementing GroupMembers, you will end up with some nasty collections in both your user and group classes. I.E. the group class will have the list of users and also a list of groupmembers which references a user and vice versa for the user class. To me this isn't that clean and will make it harder to maintain and propagate changes to the tables.
I would suggest keeping the join table in the database as you have suggested, and add a List of groups called waitingtoaccept in users and (if it makes sense too) add List of users called waitingtoaccept in groups.
These would then pull their values from your join-table in the database based on the waitingtoaccept flag.
Yes, sure you need another class like UserGroupBridge. Another good side-effect is that you can modify user membership and group members without loading potentially heavy User/Group objects to NHibernate session.
Cheers.