When i have a relation between two entities in my model:
[GroupMember] (*) ----- (1) [User]
and tries to select items from this relation with LINQ:
From entity in _user.GroupMember select entity
I always get an empty result unless I load the relation first with following statement:
_user.GroupMember.Load()
Is there a way to avoid loading the relations like this?
If you have cascading relations, you can handle them with .Include("GroupMember.AnotherTable.YetAnotherTable") which is a little nicer than having to do chained Include calls.
I just realized that when i load the User from the database, I can use Include to load GroupMember with the User like this:
Users=from entity in db.User.Include("GroupMember") select entity
But if I have several relations and maybe wants to access relations on the relations, this gets very ugly.
So I am still looking for a better/nicer solution to my issue.
Related
We're working on a CMS project with EF and MVC. We've Recently encountered a problem,
Please consider these tables:
Applications
Entities
ProductsCategories
Products
Relations are in this order:
Applications=>Entities=>ProductCategories=>Products
When we select a product by it's Id, always we should check if requested ProductsId is
just for a specific application stored in Applications table, These is for preventing load other applications products,
what is the best way to get a product for specific application id, We have two choice:
Instead of define a relation between products and applications we can do joins with productsCategories,entities, and applications to find it
=> when we want to get products we don't want to know about entities or other tables that we should join it to access applications
we can define a separate relation between products and applications and get it by simple select query
which of these is the best way and why?
Manish first thanks for your comment,Then please consider this that some of our tables does not have any relation with Entities for these tables we should define a relation with Entites to access Applications or define a separate as relation as mentioned above,For these tables we just define a relation and does not have extra work,except performance issue.still some of other tables has relations with entites so for this one defining a separat relation has extra work,
At last please consider this,in fact all of tables should access 'Entities' some by separate relation and others can access from there parents
actually for relation between products and entities we didn't define a separate relation because it doesn't has performance issue,But for relation between products and entities we should consider performance issue because in every request we should access Applications to check request Id is for current Application
So what is your idea?
Let's look at your options
Instead of defining a relationship, you can join the three tables to get the correct set of products: In this case, you won't have to make any database changes and anyway, you won't be fetching all the joined tables data, you would fetch only that data, which you have specified in your Linq Select List. But then, 3-tables join can be a little performance degrading when the number of rows will be very high at some point of time
You can define a separate relationship between the two said tables: In this case you would have to change your database structure, that would mean, making changes in your Entity and Entity Model, and lot of testing. No doubt, it will mean simple code, ease of usage which is always welcome.
So you see, there is no clear answer, ultimately it depends on you and your code environment what you want to go with, as for me, I would go for creating a separate relationship between the Application and Product entity, cause that would cause a cleaner code with a little less effort. Besides as they say, "Code around your data-structure, and not the otherway around"
I have an application that has some basic entities
Posts
posts have:
Likes
Comments
and Ratings
I then have an SQL view to query for all three. With that I have a model called something like PostActivityView. A post has an activity view so I can call
#post.activity_view
which returns a collection of the appropriate values (from Likes, Comments, and Ratings). This all works correctly.
My issue is that this returns a collection of hashmaps, not Comments, Likes, and Ratings. This makes sense because my view is creating a new "with PostEvents as (...)" result. My question: is there a way to generalize these results and represent them with an ActiveRecord object?
Likes, Comments, and Ratings have different attributes so I do some aliasing in the view (comment's have comment.body for text and Ratings can have rating.comments for text so when needed I rename something like review.comments to .body). So my results all have the same attributes. It seems like I should be able to make an ActiveRecord object like PostEvent which just has the aliased columns. Is this possible?
I don't know how to do what you're describing. However ,do you really need to store them in separate tables? You could keep them all in a single table and use single table inheritance (http://api.rubyonrails.org/classes/ActiveRecord/Base.html#label-Single+table+inheritance) to have separate classes (Likes, Comments, or Ratings) for each type of thing a particular row represents. Then the common stuff could sit up in the parent class, and the stuff specific to the more granular things could go into the descendant classes.
It sounds like your situation is the opposite of that and you're combining separate tables into a single union. I suspect that'd be very difficult to implement in ActiveRecord itself as different databases have different rules for how and when the contents of a database view may be modified (i.e., if you could somehow create an AR class that referenced your view the way you're proposing, what would happen when you call save?)
It sounds like you've gone down the path of providing a view to make it convenient to retrieve all of these objects in one set as a single type of object, when your requirement is really to bring back different objects.
Based on that I'd question the use of the view at all. I'm not anti-view you understand -- we use them quite a lot for producing read-only reports in our application for performance reasons -- but if you need the rows to be returned as their proper object type then I'd retrieve them separately as Likes, Comments, and Ratings.
First solution would be to use the gem scenic and create an activity_views view by using a union query:
create view activity_views
as (
select ...
from likes
union
select ...
from comments
union
select ...
from rating
)
your data need to be homogenous of course.
I'm a beginner in NHibernate. I have to write a complex query on say an "Employee" to populate all the associations for Employee based on the where clause. What I'm looking for is similar to this - when you do a Employee.FindById(10) should fill up OwnedDepartment, SubscribedGroups etc.
The Employee model I need to populate is really heavy (many associations with other objects)
but I need to populate only few associations. How do I achieve it using a query over? or any other approaches?
Updated
I was reading about eager loading just now, has it something to do with the loading ? In my map I have not mentioned any loading techniques, so by default all of my employee's child element are getting loaded already. There is a bunch of queries getting triggered underneath.
All the associations are lazy loaded by default. That means that the load is triggered when you access it - that's why so many queries are issued. If you want to eagerly load the data (which means either joining the tables or - rarely - doing additional select queries at once), you have to specify it in your mapping or query, depending how you fetch your data. The concept is generally called "eager fetching".
If you want to get a single Employee by ID, the standard way to do it is using session.Get<Employee>(10) - but that approach means that eager loads need to be specified in the mapping. For mapping by code it will be c.Lazy(CollectionLazy.NoLazy); for collections or c.Lazy(LazyRelation.NoProxy) for many-to-one - see here or here for details.
I prefer specifying that kind of things in the query - just where it is used, not globally for the whole entity, regardless who is fetching and what for. In LINQ provider you have FetchMany(x => x.SubscribedGroups) for collections and Fetch(x => x.OwnedDepartment) for many-to-one relations. You can find similiar options in QueryOver, if that's your choice.
I have 2 tables ATable and AATable where both have a shared Primary Key - ATable.aKey and AATable.aKey to represent a one-to-one relationship. For my Fluent mapping I have a HasOne Relationship defined within my Fluent ATableMapping, all of which works fine. However I have noticed that querying for ATable generates a 2nd query (N+1) for the child Table AATable. My understanding is that Hasone eager loads by default, and I had assumed this would be part of the query for ATable, but I may well have this wrong?
I have researched various solutions including using .Not.LazyLoad().Fetch.Join(), PropertyRef, ForeignKey but I cannot seem to resolve the n+1 so that either it is Eager loaded with 1 query, or Lazy loaded and I can fetch the child with my queries.
Has anyone had any issues with this or have an example they know to work with no n+1? Grateful for any advice.
You have two options:
Not.LazyLoad() which disables possibility to provide lazy loaded related entity and it would enforce NHB to provide corresponding subselect within original query
Use component mapping so both entities point to the same table. This is better approach as once you decided to fetch both entities together, generated queries hit only one table - not two like within first option. This is definitely better for performance.
when it comes down to increasing performance, reducing the amount of single SQL-Queries is one part of that.
Now let's assume a very basic example: i have a blog-table and a user-table. Each blog is referencing to a user by a given primary key.
A statement could be like
SELECT blog.title, blog.text, user.name FROM blog, user INNER JOIN on blog.user_id = user.id
Now my Blog-object i would like to have a $user-property which is a User-object
My Question: Are there inbuilt features within ZF2 to handle such a case? Or would i be needed to either manually map each field of the result into my objects?
Thanks in advance
No, there are no inbuilt features with ZF2 to do this - you should consider Doctrine 2 or Propel if you want that.
With Zend\Db however, you could write such SQL statements within your mapper class and then use an ArraySerializable hydrator to populate the blog entity. The blog entity's populate() could then choose to create a user object with the user data that is passed to it.