I have currently moved my blogengine over from Linq2Sql to NHIbernate.
I'm stuck at the following performance problem:
I got one table: 'Posts', that has Id, Title, PostContent, DateCreated collumns and so on.
The problem is that, when I'm creating the "Recent posts list", I don't want the whole PostContent.
In Linq2Sql you can set lazy loading on a single property, so it won't be part of the SQL query until you actually ask for the property.
I tried doing this with Fluent NHibernate, by doing this:
Map(x => x.PostContent).LazyLoad();
It didn't work out. Googling around, it seems that NHibernate doesn't support this, so my question is, how can I fix this?
Is it really not possible to lazy load my property without moving the content to a seperate table?
Thanks in advance!
Update: this capability is now available in the NHibernate trunk.
See details on Ayende's blog, where the sample is exactly the scenario you describe here.
Here is how you can achieve what you want (kind of lazy loading but not the same)
var posts = NHibernateSessionManager.Session.CreateQuery("Select p.Id as Id, p.Title as Title, p.DateCreated as DateCreated from Post p")
.SetResultTransformer(NHibernate.Transform.Transformers.AliasToBean(typeof(Post)))
.List<Post>();
What the AliasToBean is intended for is, doing selects for specific columns (usually from more than one entities) and return a strongly typed collection instead of a System.Object[].
Now if you Bean (class) is your Post class then it will popultate to that class ONLY the requested columns and nothing else.
What you will NOT be having though is a collection of NHibernate managed objects. All Posts in the returned lists are detached non controlled objects.
For doing things like "getting all recent posts" without having to get the most "heavyweight" columns of your entity while not having to create other classes to convert the data to, the above solution is kind of perfect.
Check this out for more info:
NHibernate: returning a strongly typed list having applied an aggregate function
This is not possible when PostContent is mapped to the same table, because the performance difference is not significant in 99% of the situations. When you are in the 1%, you can consider using handbuild sql instead of a orm for that case.
Lazy/Eager loading is not possible at all with linq to sql (out of the box) as far as I know.
What you can do create a different class with the data you want to select and just select that data into a new object.
Related
I'm wondering if there is a simple way to get an ID list of all the target objects relating to a source object in a coldfusion component using ORM?
I can see that you can do a collection mapping for one-to-many relationships, but I am using many-to-many relationships. I don't want to have to get the array of objects and then loop over it to get each id.
Is there any built in function or property that could do this?
I think something like the code sample below is a little too heavy since it is getting the whole query and then getting a single column from it.
valuelist( EntityToQuery( object.getRelationalFields() ).id )
Sometimes it doesn't make sense to use ORM, and this is the time. Use the good old <cfquery> for this!
I think ORMExecuteQuery may work for you, something like this:
result = ORMExecuteQuery("select id from Model as m where m.parent.id = :id", {id = 123});
Actual clause format depends on the relationship definition.
In a result you'll have the array of model PKs.
I'm trying to reproduce the HqlQuery style 'select new ObjectToProjectOut' functionality. i.e. take a list of columns returned from a query and return as a list of ObjectToProjectOut types that are instantiated using a Constructor with as many parameters as the columns in the query.
This is in effect what 'select new ObjectToProjectOut' achieves in Hql.... but clearly that's not available in SqlQuery. I think I need to set a result transform and use either PassThroughResultTransformer, DistinctRootEntityResultTransformer etc to get it to work.
Anyone know what I should use ?
ok.... after looking at the NHibernate code it seems that I was looking for AliasToBeanConstructorResultTransformer.... of course!
However I may have found an nHibernate bug. If you have the same column name returned twice from two different tables (market.name and account.name, say) then by the time nHibernate returns the array from the db to the transformer, the first occurance of 'Name' will be used for both. Nasty.
Work around is to uniquely alias. With Hql, the sql generated is heavily aliased, so this is only a bug with SqlQuery.
Grrrr. Today must be my day, also found another nHibernate bug/issue I've posted to StackOverflow for comment.
You could use the AddEntity method to fill entities from a SQL query.
Here are two examples from the NHibernate docs:
sess.CreateSQLQuery("SELECT * FROM CATS")
.AddEntity(typeof(Cat));
sess.CreateSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS")
.AddEntity(typeof(Cat));
I am fairly new to nHibernate and DDD, so please bear with me.
I have a requirement to create a new report from my SQL table. The report is read-only and will be bound to a GridView control in an ASP.NET application.
The report contains the following fields Style, Color, Size, LAQty, MTLQty, Status.
I have the entities for Style, Color and Size, which I use in other asp.net pages. I use them via repositories. I am not sure If should use the same entities for my report or not. If I use them, where I am supposed to map the Qty and Status fields?
If I should not use the same entities, should I create a new class for the report?
As said I am new to this and just trying to learn and code properly.
Thank you
For reports its usually easier to use plain values or special DTO's. Of course you can query for the entity that references all the information, but to put it into the list (eg. using databinding) it's handier to have a single class that contains all the values plain.
To get more specific solutions as the few bellow you need to tell us a little about your domain model. How does the class model look like?
generally, you have at least three options to get "plain" values form the database using NHibernate.
Write HQL that returns an array of values
For instance:
select e1.Style, e1.Color, e1.Size, e2.LAQty, e2.MTLQty
from entity1 inner join entity2
where (some condition)
the result will be a list of object[]. Every item in the list is a row, every item in the object[] is a column. This is quite like sql, but on a higher level (you describe the query on entity level) and is database independent.
Or you create a DTO (data transfer object) only to hold one row of the result:
select new ReportDto(e1.Style, e1.Color, e1.Size, e2.LAQty, e2.MTLQty)
from entity1 inner join entity2
where (some condition)
ReportDto need to implement a constructor that has all this arguments. The result is a list of ReportDto.
Or you use CriteriaAPI (recommended)
session.CreateCriteria(typeof(Entity1), "e1")
.CreateCriteria(typeof(Entity2), "e2")
.Add( /* some condition */ )
.Add(Projections.Property("e1.Style", "Style"))
.Add(Projections.Property("e1.Color", "Color"))
.Add(Projections.Property("e1.Size", "Size"))
.Add(Projections.Property("e2.LAQty", "LAQty"))
.Add(Projections.Property("e2.MTLQty", "MTLQty"))
.SetResultTransformer(AliasToBean(typeof(ReportDto)))
.List<ReportDto>();
The ReportDto needs to have a proeprty with the name of each alias "Style", "Color" etc. The output is a list of ReportDto.
I'm not schooled in DDD exactly, but I've always modeled my nouns as types and I'm surprised the report itself is an entity. DDD or not, I wouldn't do that, rather I'd have my reports reflect the results of a query, in which quantity is presumably count(*) or sum(lineItem.quantity) and status is also calculated (perhaps, in the page).
You haven't described your domain, but there is a clue on those column headings that you may be doing a pivot over the data to create LAQty, MTLQty which you'll find hard to do in nHibernate as its designed for OLTP and does not even do UNION last I checked. That said, there is nothing wrong with abusing HQL (Hibernate Query Language) for doing lightweight reporting, as long as you understand you are abusing it.
I see Stefan has done a grand job of describing the syntax for that, so I'll stop there :-)
and have it map to a strongly typed object?
Suppose I have a entity Blog with Id,Name,Posted,IsUglyFace,YerMom
but I just only want/need Name and Posted columns. I tried to do something like:
IList blogs = repository.SimpleQuery("select Name, Posted from Blog");
This dumps out a ton more SQL and gives back an error, could not map to given object.
See http://ayende.com/Blog/archive/2007/08/26/Partial-Object-Queries-With-NHibernate.aspx
Bonus: http://ayende.com/Blog/archive/2007/08/26/Lazy-Property-Loading-In-NHibernate.aspx
(sorry for my English)
For example, in my DAL,I have an AuthorDB object, that has a Name
and a BookDB object, that has a Title and an IdAuthor.
Now, if I want to show all the books with their corresponding author's name, I have to get a collection of all the Books, and for each of them, with the IdAuthor attribute, find the Author's name. This makes a lot of queries to the database, and obviously, a simple JOIN could be used.
What are my options? Creating a 'custom' object that contains the author's name and the title of the book? If so, the maintenance could become awful.
So, what are the options?
Thank you!
Don't write something buggy, inefficient, and specialized ... when reliable, efficient, and generic tools are available. For free.
Pick an ORM. NHibernate, ActiveRecord, SubSonic, NPersist, LinqToEF, LinqToSQL, LLBLGenPro, DB4O, CSLA, etc.
You can create a View in the database that has the join built into it and bind an an object to that, e.g. AuthorBooksDB. It doesn't create too bad a maintenance headache since the view can hide any underlying changes and remains static.
If you can separate the database query from the object building, then you could create a query to get the data you need. Then pass that data to your builder and let it return your books.
With Linq to SQL, that can be done as easily as:
public IEnumerable<Book> AllBooks()
{
return from book in db.Books
join author in db.Authors on book.AuthorId equals author.Id
select new Book()
{
Title = book.Title,
Author = author.Name,
};
}
The same can be achieved with DataTables / DataSets:
public IEnumerable<Book> AllBooks()
{
DataTable booksAndAuthors = QueryAllBooksAndAuthors(); // encapsulates the sql query
foreach (DataRow row in booksAndAuthors.Rows)
{
Book book = new Book();
book.Title = row["Title"];
book.Author = row["AuthorName"];
yield return book;
}
}
Thank you very much for your inputs.
Actually, we are trying to keep the database objects as close as possible to the actual columns of the corresponding table in the database. That's why we cannot really add a (string) 'Author' attribute to the BookDB object.
Here is the problem I see with using 'View' objects. In the database, if the schema has to be modified for any reason (e.g: In the Book table, the 'Title' column has to be modified for 'The_Title', how do we easily know all the 'View' objects that have to be modified? In other words, how to I know what objects have to be modified when they make queries that use multiple joins?
Here, since we have a AuthorsBooks object, we can see by the name that it probably makes a query to the book and author tables. However, with objects that make 4 or 5 joins between tables we cannot rely on the object name.
Any ideas? (Thank you again, this is a great site!)
I suggest you take a look at Domain Driven Design. In DDD, you get all business objects from a repository. The repository hides your data store and implementation, and will solve your problems on how to query data and keeping track of database changes. Because every business object is retrieved from the repository, the repository will be your single point of change. The repository can then query your database in any way you find efficient, and then build your domain objects from that data:
var books = new BookRepository().GetAllBooks();
You should be able to code the repositories with any of the technologies mentioned by Justice.