Some quick nhibernate problem:
I have sql tables:
Item { Id, Name }
ItemRange { Id, Name }
ItemHasItemRange { Id, ItemId, ItemRangeId }
Mappings are simple, so I will not paste them, the ItemId and ItemRangeId are foreign keys, Item class has ItemHasItemRanges collection mapped as lazy bag.
I want all items which are in particular ItemRange, but I do not want to retrieve associated ItemRangeObjects, I just want to do inner join to narrow results.
When I do it like that:
c.CreateCriteria("Item", "i")
.CreateAlias("ItemHasItemRanges", "ihpr", JoinType.InnerJoin)
.Add(Restrictions.Eq("ihpr.ItemRange.Id", I18nHelper.CurrentItemRange.Id));
It works fine, but all ItemHasItemRange objects are fetched as well to the Item.ItemHasItemRanges collections (which is mapped as lazy)
I do not want to fetch Item.ItemHasItemRanges, because it takes time. I just want to do inner join to limit result set. It is possible in NHibernate?
So I think that you just want to retrieve those objects in order to show an overview / list, and you are not going to actually 'do' something with those objects (unless perhaps loading one of them) ?
In that case, I think that it is better for you to work with 'projections'.
Here's the scenario:
You'll have to create a (simple) class that just contains the properties that you want to show (where you're interested in).
You'll have to 'import' that class into NHibernate, so that NHibernate knows of its existence.
Next, you can create your Criteria statement like you have it now. (Working with your domain classes).
Then, you should specify how the projection should look like. That is, how the properties of your Item entity map to the properties of your 'DTO'/View class (= the simple class you just created).
Specify that an AliasToBean ResultTransformer should be used.
Then, execute your Criteria query. NHibernate will be able to produce the simplest possible query that is needed in order to retrieve all the data that is necessary.
I've explained something similar here
I find out the problem was somewhere else. ItemHasItemRange table did not have multiple index on ItemId and ItemRangeId - id only had separate indexes on each field. Thats why performance was so poor.
But NHibernate question is still valid - is it possible to create inner join for criteria only to narrow results and not to fetch all joined objects which normally are lazy.
Related
In our webapp we have a number of places where you would be updating a number of tables in one complex form/view. In raw SQL I would probably select a bunch of columns from a bunch of tables and edit that one record on the primary table as well as related parent/child tables.
In hibernate I would probably just pull a JPA entity for the main table and let hibernate fetch the parent/child relationships as I populate the view. And then later pull from my view back to the entity and call entitymanger .perist/merge.
In JOOQ I have a number of options but it appears you can pull a main record via selectFrom/fetch then use fetchChild fetchParent to pull typed related records like so...
LoadsRecord load = dslContext.selectFrom(LOADS)
.where(LOADS.ID.eq(id))
.fetchOne();
SafetyInspectionsRecord safetyInspection = load.fetchParent(Keys.LOADS__FK_SAFETY_INSPECTION);
So this way I am able to pull related records in a typesafe manner. The only annoying thing is I have to run another full query every time I call fetchParent or fetchDhild. Is there a way to eagerly fetch these all at once to avoid multiple round trips to the DB?
It is really nice to have these classes like LoadsRecord for CRUD screens, it makes updating the DB easy.
Classic approach using joins
There are various ways you can achieve materialising a to-one relationship. The simplest one being a simple JOIN or LEFT JOIN if the relationship is optional.
E.g.:
Result<?> result =
ctx.select()
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
You probably want to work with the generated records thereafter, so you can use various mapping tools to map the generic Record types to the two UpdatableRecord types for further CRUD:
for (Record r : result) {
LoadsRecord loads = r.into(LOADS);
SafetyInspectionsRecord si = r.into(SAFETY_INSPECTIONS);
}
Using nested records
Starting from jOOQ 3.15 and #11812, MULTISET and ROW operators can be used to create nested collections and records. So, in your query, you could write:
Result<?> result =
ctx.select(
row(LOADS.ID, ...),
row(SAFETY_INSPECTIONS.ID, ...))
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
That would already help map the nested data structures into the desired format. Starting from jOOQ 3.17 and #4727, you can even use table expressions directly to generate nested records:
Result<Record2<LoadsRecord, SafetyInspectionsRecord>> result =
ctx.select(LOADS, SAFETY_INSPECTIONS)
.from(LOADS)
.join(SAFETY_INSPECTIONS)
.on(LOADS.SAFETY_INSPECTIONS_ID.eq(SAFETY_INSPECTIONS.ID))
.fetch();
This new feature is definitely going to close one of jOOQ's biggest gaps. You could even simplify the above using implicit joins to this:
Result<Record2<LoadsRecord, SafetyInspectionsRecord>> result =
ctx.select(LOADS, LOADS.safetyInspections())
.from(LOADS)
.fetch();
I tried to do a lot of research but I'm more of a db guy - so even the explanation in the MSDN doesn't make any sense to me. Can anyone please explain, and provide some examples on what Include() statement does in the term of SQL query?
Let's say for instance you want to get a list of all your customers:
var customers = context.Customers.ToList();
And let's assume that each Customer object has a reference to its set of Orders, and that each Order has references to LineItems which may also reference a Product.
As you can see, selecting a top-level object with many related entities could result in a query that needs to pull in data from many sources. As a performance measure, Include() allows you to indicate which related entities should be read from the database as part of the same query.
Using the same example, this might bring in all of the related order headers, but none of the other records:
var customersWithOrderDetail = context.Customers.Include("Orders").ToList();
As a final point since you asked for SQL, the first statement without Include() could generate a simple statement:
SELECT * FROM Customers;
The final statement which calls Include("Orders") may look like this:
SELECT *
FROM Customers JOIN Orders ON Customers.Id = Orders.CustomerId;
I just wanted to add that "Include" is part of eager loading. It is described in Entity Framework 6 tutorial by Microsoft. Here is the link:
https://learn.microsoft.com/en-us/aspnet/mvc/overview/getting-started/getting-started-with-ef-using-mvc/reading-related-data-with-the-entity-framework-in-an-asp-net-mvc-application
Excerpt from the linked page:
Here are several ways that the Entity Framework can load related data into the navigation properties of an entity:
Lazy loading. When the entity is first read, related data isn't retrieved. However, the first time you attempt to access a navigation property, the data required for that navigation property is automatically retrieved. This results in multiple queries sent to the database — one for the entity itself and one each time that related data for the entity must be retrieved. The DbContext class enables lazy loading by default.
Eager loading. When the entity is read, related data is retrieved along with it. This typically results in a single join query that retrieves all of the data that's needed. You specify eager loading by using the Include method.
Explicit loading. This is similar to lazy loading, except that you explicitly retrieve the related data in code; it doesn't happen automatically when you access a navigation property. You load related data manually by getting the object state manager entry for an entity and calling the Collection.Load method for collections or the Reference.Load method for properties that hold a single entity. (In the following example, if you wanted to load the Administrator navigation property, you'd replace Collection(x => x.Courses) with Reference(x => x.Administrator).) Typically you'd use explicit loading only when you've turned lazy loading off.
Because they don't immediately retrieve the property values, lazy loading and explicit loading are also both known as deferred loading.
Think of it as enforcing Eager-Loading in a scenario where your sub-items would otherwise be lazy-loading.
The Query EF is sending to the database will yield a larger result at first, but on access no follow-up queries will be made when accessing the included items.
On the other hand, without it, EF would execute separte queries later, when you first access the sub-items.
include() method just to include the related entities.
but what happened on sql is based on the relationship between those entities which you are going to include what the data you going to fetch.
your LINQ query decides what type of joins have to use, there could be left outer joins there could be inner join there could be right joins etc...
#Corey Adler
Remember that you should use .Include() and .ThenInclude() only when returning the object (NOT THE QUERYABLE) with the "other table property".
As a result, it should only be used when returning APIs' objects, not in your intra-application.
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.
We are using Oracle 10g database, NHibernate, WCF and Silverlight 3.0 in our project
The situation we have is that the entities in my project have many properties. But for certain situations, like showing the options in dropdown, I only want to retrieve the list of ID and Name field for that entity. I do not want to return a list of the entire entity object as a whole as there are many columns in the table. Presently I am using two SELECT queries: one to fetch the list of IDs and second to fetch the list of Names separately. Then I join these two queries and form a Dictionary and pass it to the UI.
The concern for me is that would it be possible to achieve this in a single query itself?
One approach that I know of is to create a new class having only the ID and Name property, import it into NHiberante and then form a list of this new class and send it to the UI. I want to avoid this approach for now as there are many tables for which I have to implement this functionality and hence I will have to create many new classes and corresponding xml files.
Any sort of help would be greatly appreciated.
Here is one way to do it using the Criteria API, Projections and AliasToBean. If it's a simple non-persistent class containing Id and Name you can reuse the class. NHibernate query CreateCriteria
I have an entity with several Many to One relationships and I've found I can eagerly fetch them in one query like such:
public Accommodation GetEager(int id)
{
IList<Accommodation> query = NHibernateSession.CurrentFor(NHibernateSession.DefaultFactoryKey)
.CreateQuery(#"
select a from Accommodation as a
left join fetch a.AccommodationType
left join fetch a.AccommodationUnitType
left join fetch a.CollectionType
where a.Id = :id
")
.SetProperties(new {id})
.SetCacheable(true)
.List<Accommodation>();
return query.SingleOrDefault();
}
However, the relationships don't always exist, and I've defined the mappings like so:
mapping.References(x => x.AccommodationUnitType).NotFound.Ignore();
I've found that when a relationship doesn't exist, NHibernate generated a second SQL query looking for it, presumably because it's found that the property is null.
My first question is, how can I prevent this second sql query?
My second question is, is there an easier way to do this fetching into one query? It seems very basic behaviour that one would want to fetch everything in one query rather than a seperate query for each many-to-one relationship (which seems to be the default behaviour).
Are you sure you're using NotFound().Ignore() correctly? This setting determines what NHibernate will do if there is an invalid foreign key. In that case NotFound().Ignore() prevents throwing an EntityNotFoundException. With the setting, if the related object is not found then the value will be null. If you have referential integrity for this relationship then you do not need NotFound().Ignore().
The behavior you're seeing is apparently expected and well known and unlikely to change.
As for your second question, I would advise always starting with the default behavior of lazy loading and only optimizing with eager loads as needed for real world performance problems. Lazy loading is frequently more efficient than eager fetching because a) the object may already be in cache (no db trip required) and b) selecting by primary key is very fast. It's very possible that your query with three joins performs worse than four selects by primary key.