I've got a situation where I need to prefetch some entities through a many-to-many relationship. So it's like the classic BlogPost <- BlogPostTag -> Tag situation.
Yes, I'm aware of LoadOptions but I can't use it because it's a web application and I'm using the one datacontext per request pattern.
It also seems you can't use projection to prefetch many-to-many relationships. Yes? No?
I want to return IQueryable<Tag> based on a set of Blogs. The best I can do is get it to return IQueryable<IEnumerable<Tag>> by doing the following:
public IQueryable<Tag> GetJobsCategories(IQueryable<BlogPost> blogPosts)
{
var jobCats = from bp in blogPosts
select bp.BlogPostTags.Select(x => x.Tag);
return jobCats;
}
Can I flatten that? Am I missing something obvious? Is there another approach I can take?
And no, I can't change ORMs ;-)
This will work, you can just drill down in the linq query
public IQueryable<Tag> GetJobsCategories(IQueryable<BlogPost> blogPosts)
{
return from bp in blogPosts
from tag in bp.BlogPostTags
select tag;
}
If you declare the method as such:
public static IQueryable<Tag> GetJobsCategories(this IQueryable<BlogPost> blogPosts)
You can use it as extension method on queryables. For instance
myContext.BlogPosts.GetJobsCategories()
Related
I'm tinkering with Neo4j 2.3.0 and Laravel 5.1 using NeoEloquent. I've set up a couple of dummy nodes and some relationships between them:
image of Neo4j model - apologies, I cannot insert images directly yet :)
So articles can use a template. The inverse of this relationship is that a template is used by an article.
I've set up the classes like so:
Class Template extends Model
{
public function articles()
{
return $this->hasMany('App\Article', 'USED_BY');
}
}
And:
Class Article extends Model
{
public function template()
{
return $this->belongsTo('App\Template', 'USES');
}
}
So far, so good, I think.
I have a page where I am wanting to eventually list all of the articles in the system, along with some useful metadata, like the template each ones uses. For this, I have set something up in the controller:
$articles = array();
foreach (Article::with('template')->get() as $article) {
array_push($articles, $article);
}
return $articles;
Not the most elegant, but it should return the data for both the article and it's associated template. However:
[{"content":"Some test content","title":"Test Article","id":28,"template":null},{"content":"Some flibble content","title":"Flibble","id":31,"template":null}]
So the question is - why is this returning null?
More interestingly, if I set up the relationship to the same thing in BOTH directions, it returns the values. i.e. if I change the USED_BY to USES, then the data is returned, but this doesn't make sense from an architectural point of view - a template does not 'use' an article.
So what am I missing?
More interestingly, if I set up the relationship to the same thing in BOTH directions, it returns the values.
That's correct, because this is how it operates. It is worth knowing that the relationship methods you have defined represent the relationship itself, which means for both models Template and Article to target the USED_BY relationship from any side it has to be the same in articles() and template.
The solution would be to use something like USES (or any notion you like) on both sides. This reference should help you make good decisions regarding your relationships.
On the other hand, if you still wish to have different relations on the sides then kindly note that in your model (image) both relationships are in outgoing direction. i.e. Fibble-[:USES]->Template and Template-[:USED_BY]->Fibble which means template() should be an outgoing relationship such as hasOne instead of belongsTo which is incoming.
Which is the best approach for updating multiple records at a time using ASP.NET Web API and OData against an Entity Framework database?
Let's suppose we have a table in our database with a "State" field, and we want to do something similar to this SQL statement: "UPDATE Table set State = 1". Should I do a loop with GET and PUT for each record? I do not like it at all, I guess it must be a better way to accomplish this.
Thank you
It looks like you can't do this natively in OData, but there is one approach that will definitely work; just use a native Web API action to perform the update.
E.g.
[HttpPut]
[Route("resources/state")]
public IHttpActionResult UpdateState(int newState)
{
db.Database.SqlCommand("UPDATE Table SET State = #p0", newState);
}
You'd call this from a client using a PUT /resources/state?newState=1.
It might be clearer to make newState an enum:
public enum State
{
New = 0,
Processed = 1,
Error = 2,
etc.
}
[HttpPut]
[Route("resources/state")]
public IHttpActionResult UpdateState(State newState)
{
db.Database.SqlCommand("UPDATE Table SET State = #p0", (int)newState);
}
Then your call becomes PUT /resources/state?newState=Processed which is a little clearer.
OData supports such kinds of operations. You can use OData Action to update the state.
Blog: odata-actions-and-functions
Sample: ODataActionSample
Ok!
I have to say both technology are great. Although there seems that something I do not get it.
You have a data in you database (and let say you want to show data from a table that has references to other tables).
I have a model with List or IEnumerable or IQueryable or whatever...
So in my view I want do foreach through the list of object and take advantage of cool feature of references to other tables. No problem in controller while you are in
using (var datatabse = new MyEntity)
{
}
But when you get out of using db has disposed and you get common error The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
So i do not see other options as creating in memory copies of entity objects...but you loose all cool EF4 references and you have to manually load data first in your model and then with foreach show it on the view.
So instead of List<(EF4Type)> or IEnumerable<(EF4Type)> or IQueryable<(EF4Type)>
you have to do List<(MyCustomHelperClass)> where MyCustomHelperClass represents a class with properties similiar to entity objects and probably some additional beacuse you do not have access to properties of referenced tables Then you have to do foreach and Load data into this List and the another #foreach on the view with Razor to show all.
Twice as much work and if project is big...you can see a bigger picture of how manny those helperClasses you need. Was all this cool new technology really meant to be used in that way?....or am I missing something.
You are probably getting that error when you reference a lazy loaded property in your view. You should eager load everything you need in the Controller before passing it to the View.
See Loading Related Objects (Entity Framework).
The following example will cause all courses to be retrieved with the departments in the same query. This is eager loading.
// Load all departments and related courses
var departments1 = context.Departments
.Include(d => d.Courses)
.ToList();
Without the Include() part, courses could be retrieved later (possibly after your context has been disposed in the view). This is called lazy loading.
Along with eager loading as remembered by jrummell, there's also another way of loading related entries, it's explicit loading. Let's suppose you have a User entity, with many Groups entities related to it. You can explicitly load them:
var user = context.Users.Find(id); // Load the user.
context.Entry(user)
.Collection(u => u.Groups)
.Load();
This way you don't have to use the .Include(), and you can even filter the Groups:
context.Entry(user)
.Collection(u => u.Groups)
.Query()
.Where(g => g.SomeProperty.Contains("something"))
.Load();
TheMentor,
Depending on whether you have a repository or a db context, this object should only live for the duration of the controller action (Request), so you should be able to do everything required within the confines of the action.
Maybe i've misunderstood, but based on your question, this is what your issue appears to be. If I have misunderstood, then I'd still suggest that the db repository or db context should be referenced across the controller, rather then invoking it inside the action each time.
so you should see something like this in your controller:
public class TasksController : BaseController
{
private readonly TaskService _serviceTasks;
public TasksController(IRepository repository)
{
_serviceTasks = new TaskService(repository);
}
//
// GET: /Tasks/
public ActionResult Index()
{
var viewModel = _serviceTasks.All<Task>();
return View(viewModel);
}
public ActionResult Details(int id)
{
var domainModel = _serviceTasks.GetById<Task>(id);
var viewModel = PopulateDetailsViewModel(domainModel);
return View(viewModel);
}
//.. rest of actions cut
}
I'd like to include some additional functions in my Doctrine 2 entities to contain code that I'm going to have to run quite frequently. For example:
User - has many Posts
Post - has a single user
I already have a function $user->getPosts(), but this returns all of my posts. I'm looking to write a $user->getActivePosts(), which would be like:
$user->getPosts()->where('active = true') //if this were possible
or:
$em->getRepository('Posts')->findBy(array('user'=>$user,'active'=>true)) //if this were more convenient
As far as I can tell, there's no way to get back to the entity manager though the Entity itself, so my only option would be
class User {
function getActivePosts() {
$all_posts = $this->getPosts();
$active_posts = new ArrayCollection();
foreach ($all_posts as $post) {
if ($post->getActive()) {
$active_posts->add($post);
}
}
return $active_posts;
}
However, this requires me to load ALL posts into my entity manager, when I really only want a small subset of them, and it requires me to do filtering in PHP, when it would be much more appropriate to do so in the SQL layer. Is there any way to accomplish what I'm looking to do inside the Entity, or do I have to create code outside of it?
I think you should implement the method on the PostRepository rather than on the entity model.
I try to keep all model related logic in the repositories behind "domain specific" methods. That way if you change the way you represent whether a post is active or not, you only have to change the implementation of a single method instead of having to find all the active = true statements scattered around in your application or making changes in an "unrelated" entity model.
Something like this
PostRepository extends EntityRepository {
public function findActiveByUser($user){
// whatever it takes to get the active posts
}
}
I have a basic view that returns the same columns as a table (give or take 1 field)
in my DAL code, i am returning a list of MyTableObject, however in some cases, i will call the view to return the same data, but from different sources.
List<MyTableObject> tableObjects = new List<MyTableObject>();
if (case1)
tableObjects = entities.MyTableObjects.Where(criteria).ToList();
else
tableObjects = entities.MyViewObjects.Where(criteria).ToList(); // <-- This will obviously break
return tableObjects;
is there a way to Map view entities to be returned as table entities? (other than having table and view implement the same interface and return that interface) i would like to keep the return type as MyTableObject.
I came across Auto Mapper, but not sure if it would be suitable for this scenario..
Looks like i found a cool solution to this..
Initially I tried to implement interface approach and run into some casting issues (using interfaces alongside my predicate builder), and also with interfaces having to create partial classes for each entity that implement the interface..
the answer.. POCOs.
Iused Poco Template for EF, and than simply edited xxxPocoGenerator.Context.tt to return MyTable object from MyViews collection (one line).
public ObjectSet<Trade> v_Trade {
get { return _v_Trade ?? (_v_Trade = CreateObjectSet<Trade>("Trades")); }
}
nice and easy..
You can write a stored procedure (or CommandText in the model, without creating DB Object) that will simply call "Select * from View". Then create Function Import for this procedure and set the return type to MyTableObject.