I have a composite type of model for my table.
The table model contains several row models as member variable.
Each row model gets the table model passed in the constructor and stores it as member variable.
Now the problem comes up when I need to detach the models. In my table model I need to call detach for each of the row models. And in each row model I need to detach the table model. This obviously leads in circular detaching and eventual stack overflow.
How can I properly detach the models with this design?
As already mentioned, you should use Component's onDetach() to detach your models. Then you should be able to avoid cyclic detaching easily.
Related
I am trying to set up multiple DbContext objects in my solution. One will be the initialization context and the others will be bounded contexts.
The scenario I am having difficulty resolving is as follows:
Having two database tables, a parent object and a child object. Upon initialization the tables are created okay with all the properties on, however in the bounded context I want to use the parent object and only return a subset of properties from the child object.
At the moment both the initialization context and the bounded context use the same parent class so therefore I can't use a third info object for the child object in the bounded context.
Would I therefore need to create another parent object for use in the bounded context which uses another child object with subset of properties?
So would I need in the initialization context a parent and child object and in the bounded context a separate parent and child object where the child object only has a subset of properties and the parent object points to that child?
I am quite happy to create the child class with a subset of properties but it seems a bit long winded to create a separate parent object to point to the child info object but I can't see a way around this.
Having watched Julie Lermans TechEd Europe 2014 "Entity Framework Model Partitioning in Domain-Driven Design Bounded Contexts" https://www.youtube.com/watch?v=rGA_FNew-6g I have decided to go down the route of having a master context purely for database migrations.
I found the idea of using events and message queues and separating the data into different tables in the database interesting but it's probably a bit heavy handed for our needs at the moment.
In my core data object model I have two entities that are related with a one to many relationship.
Example:
table A can have many table B's associated with it.
My question is, when I add table B rows (with the object reference to Table A), do I need to independently save table B, or will the save of Table A also persist table B?
It appears that table B is being persisted when table A is saved.. but I cannot find any documentation to confirm this.. and I want to make sure I don't run the risk of losing data.
(Core Data saves are very slow.. and saving Table B data independently is a hog).
Don't think about tables. What gets saved is the managed object context, which translates into: any changes you've made to managed objects using that context.
I need to copy data from one database to another with EF. E.g. I have the following table relations: Forms->FormVersions->FormLayouts... We have different forms in both databases and we want to collect them to one DB. Basically I want to load Form object recursively from one DB and save it to another DB with all his references. Also I need to change IDs of the object and related objects if there are exists objects with the same ID in the second database.
Until now I have following code:
Form form = null;
using (var context = new FormEntities())
{
form = (from f in context.Forms
join fv in context.FormVersions on f.ID equals fv.FormID
where f.ID == 56
select f).First();
}
var context1 = new FormEntities("name=FormEntities1");
context1.AddObject("Forms", form);
context1.SaveChanges();
I'm receiving the error: "The EntityKey property can only be set when the current value of the property is null."
Can you help with implementation?
The simplest solution would be create copy of your Form (new object) and add that new object. Otherwise you can try:
Call context.Detach(form)
Set form's EntityKey to null
Call context1.AddObject(form)
I would first second E.J.'s answer. Assuming though that you are going to use Entity Framework, one of the main problem areas that you will face is relationship management. Your code should use the Include method to ensure that related objects are included in the results of a select operation. The join that you have will not have this effect.
http://msdn.microsoft.com/en-us/library/bb738708.aspx
Further, detaching an object will not automatically detach the related objects. You can detach them in the same way however the problem here is that as each object is detached, the relationships that it held to other objects within the context are broken.
Manually restoring the relationships may be an option for you however it may be worthwhile looking at EntityGraph. This framework allows you to define object graphs and then perform operations such as detach upon them. The entire graph is detached in a single operation with its relationships intact.
My experience with this framework has been in relation to RIA Services and Silverlight however I believe that these operations are also supported in .Net.
http://riaservicescontrib.codeplex.com/wikipage?title=EntityGraphs
Edit1: I just checked the EntityGraph docs and see that DetachEntityGraph is in the RIA specific layer which unfortunately rules it out as an option for you.
Edit2: Alex Jame's answer to the following question is a solution to your problem. Don't load the objects into the context to begin with - use the notracking option. That way you don't need to detach them which is what causes the problem.
Entity Framework - Detach and keep related object graph
If you are only doing a few records, Ladislav's suggestion will probably work, but if you are moving lots of data, you should/could consider doing this move in a stored procedure. The entire operation can be done at the server, with no need to move objects from the db server, to your front end and then back again. A single SP call would do it all.
The performance will be a lot better which may or may not not matter in your case.
I'm building an iPad application where I need user to create entity dynamically. I'm already having 3 entities which program uses.
Could you help me with code how to do it?
I want to understand the whole structure according to my understanding I have to create new managedObjectModel, add new entities and than merge it with existing one, is it correct?
While it is possible to create a new entity and a new model on the fly in practice this is massively complex. If nothing else you would have to migrate any existing persisted data to the new model and a new persistent store file. I strongly recommend against attempting this especially if you are just starting out with Core Data.
You do have options:
Firstly, are you sure you actually need a new entity? People just starting out with Core Data often mistake entities for managed objects. Entities are to managed objects as classes are to instances. Entities are abstractions used to create the object graph. They don't actually contain data. The times when you need new entities are very,very rare.
Secondly, if you do need some kind of dynamic entity, it would usually be best to decompose the dynamic entity into numerous fixed subentities and then use relationships to create a virtual entity. E.g. you need a dynamic Person "entity" so you create several entities in the model each of which holds one attribute of the person. You could have a Field entity which would have a fieldName attribute and then a fieldValue attribute. Then have a an actual Person entity that has no attributes but just relationships to the necessary Field objects. You could add any fields needed to any person and then reconstitute an virtual person object by walking the relationships to its fields.
I rather doubt however that you need that kind of flexibility. Such a need is very rare. I would step back and see exactly what dynamic data you think the user might need to enter.
That's correct -- you'd create an array of NSEntityDescription objects, then call setEntities: on the new managed object model. Then, finally, you'd merge that model with your built-in model.
But note that you can't change a model once it has been used to create a managed object context (or used for storage). You'll need to create new storage and context after the model is changed.
For part of the project I'm currently working on, I have a set of four tables for syndicatable actions. One table is the abstract base for the other three, and each table is represented in my EF model like so:
EF Model -- Actions http://chris.charabaruk.com/system/files/images/EF+Model+Actions.png
There are two problems that I'm currently facing with this, however. The first problem is that Actor (a reference to a User) and Subject (a reference to an entity of the class associated with each type of action) are null in my subclasses, despite the associated database columns holding valid keys to rows in their associated tables. While I can get the keys via ActorReference and SubjectReference this of course requires setting up a new EF context and querying it for the referenced objects (as FooReference.Value is also null).
The second problem is that the reciprocal end of the relationship between the concrete action classes and their related entity classes always turn up nothing. For example, Task.RelatedActions, which should give me all TaskAction objects where Subject refers to the particular task object on which RelatedActions is called, is entirely devoid of objects. Again, valid rows exist in the database, Entity Framework just isn't putting them in objects and handing them to me.
Anyone know what it is I'm doing wrong, and what I should do to make it work?
Update: Seems that none of the relationship properties are working in my entity model any more, at all. WTF...
I think the issue you are experiencing here is that by default the EF does not automatically load related entities. If you load an entity, the collection or reference to related entities will be empty unless you do one of the following things:
1) Use eager loading in order to retrieve your main entity and your related entity in a single query. To do this, modify your query by adding a call to the Include method. In your sample above, you might use the following query:
from a in context.Actions.Include("Actor") select a
This would retrieve each of the actions with the related Actor method.
2) Use explicit lazy loading to retrieve the related entity when you need it:
action1.ActorReference.Load()
In the version of the EF which will ship with .Net 4.0, you will also have the following additional option:
3) Turn on implicit lazy loading so that related entities will automatically be retrieved when you reference the navigation property.
Danny