I have a project using NH 3.1 and have been using the QueryOver syntax for everything thus far.
One aspect of this project lives in a organization-wide database that I have read-only access to and is using a completely differently DBMS (Oracle vs MSSQL). So I store references from my objects (Foos) to their objects (Bars) using a standard many-to-many table
FooBars
FooID int not null PK
BarID int not null PK
And my domain object, instead of having a Iset<Bar> instead has an ISet<int> BarIDs which is manually mapped to the FooBars table. This prevents NH from trying to do the impossible and join all the way over to the Bars table (I can use a BarRepository.Get() to retrieve the details of the Bars later, if I need them, and in this case, I wouldn't, because I just need the IDs to filter the list of objects returned).
Given IList<int> SelectedBars how can I write a QueryOver<Foo> where BarIDs contains any element in SelectedBars?
SQL something like
...FROM foos INNER JOIN foobars on foo.fooID = foobars.fooID WHERE barID IN ( ... )
It is not possible with QueryOver. Two years ago, I had a similar question about filtering value collections. (Note: QueryOver is based on Criteria API).
I'm not 100% sure, but it probably works with HQL. It is much more powerful.
You may include an SQL statement into the QueryOver criteria.
I don't really understand why you don't map it as a list of entities. There is lazy loading to avoid unnecessary loading - although there are some trade offs sometimes. You can access the ID of NH proxies without hitting the database. Mapping ids makes usually life much harder.
Try:
session.QueryOver<Foo>()
.JoinQueryOver(x => x.FooBars)
.WhereRestrictionOn(x => x.BarId).IsIn( ... )
So 3 years later, I'm back to report how I did solve this.
public class Foo :Entity {
public virtual ISet<FooBar> BarIDs { get; protected internal set; }
} ...
public class FooBar :Entity {
protected internal FooBar() { }
protected internal FooBar(Foo f, int BarID) { ... }
public virtual Foo Foo { get; protected internal set; }
public virtual int BarID { get; protected internal set; }
}
This is basically what Stefan suggested, and what's hinted at in the related post. You just have to eat the overhead of writing an extra entity and referencing it. Remember that I'm storing BarIDs instead of full Bar objects, because I'm dealing with a hard boundary between two databases: the Bars are stored in an entirely different database on a different platform than the Foos. Otherwise of course, you're far better off just telling Foo that is has an ISet<Bar>.
Finding Foos by SelectedBarIDs is then easy, much like Thilak suggested:
session.QueryOver<Foo>().JoinQueryOver<FooBar>(f => f.BarIDs).
WhereRestrictionOn(b => b.BarID).IsIn(...)...
It's an interesting problem, working across the database boundary like this. I can't say I like having to do it, but if someone else is going to take the time to maintain a list of Bars and make it available for my use, it would be a giant waste of resources for me to do the same. So a small inefficiency in having the wrapper class is a very easy cost to justify.
Related
In my domain I have something called Project which basically holds a lot of simple configuration propeties that describe what should happen when the project gets executed. When the Project gets executed it produces a huge amount of LogEntries. In my application I need to analyse these log entries for a given Project, so I need to be able to partially successively load a portion (time frame) of log entries from the database (Oracle). How would you model this relationship as DB tables and as objects?
I could have a Project table and ProjectLog table and have a foreign key to the primary key of Project and do the "same" thing at object level have class Project and a property
IEnumerable<LogEntry> LogEntries { get; }
and have NHibernate do all the mapping. But how would I design my ProjectRepository in this case? I could have a methods
void FillLog(Project projectToFill, DateTime start, DateTime end);
How can I tell NHibernate that it should not load the LogEntries until someone calls this method and how would I make NHibernate to load a specifc timeframe within that method?
I am pretty new to ORM, maybe that design is not optimal for NHibernate or in general? Maybe I shoul design it differently?
Instead of having a Project entity as an aggregate root, why not move the reference around and let LogEntry have a Product property and also act as an aggregate root.
public class LogEntry
{
public virtual Product Product { get; set; }
// ...other properties
}
public class Product
{
// remove the LogEntries property from Product
// public virtual IList<LogEntry> LogEntries { get; set; }
}
Now, since both of those entities are aggregate roots, you would have two different repositories: ProductRepository and LogEntryRepository. LogEntryRepository could have a method GetByProductAndTime:
IEnumerable<LogEntry> GetByProductAndTime(Project project, DateTime start, DateTime end);
The 'correct' way of loading partial / filtered / criteria-based lists under NHibernate is to use queries. There is lazy="extra" but it doesn't do what you want.
As you've already noted, that breaks the DDD model of Root Aggregate -> Children. I struggled with just this problem for an absolute age, because first of all I hated having what amounted to persistence concerns polluting my domain model, and I could never get the API surface to look 'right'. Filter methods on the owning entity class work but are far from pretty.
In the end I settled for extending my entity base class (all my entities inherit from it, which I know is slightly unfashionable these days but it does at least let me do this sort of thing consistently) with a protected method called Query<T>() that takes a LINQ expression defining the relationship and, under the hood in the repository, calls LINQ-to-NH and returns an IQueryable<T> that you can then query into as you require. I can then facade that call beneath a regular property.
The base class does this:
protected virtual IQueryable<TCollection> Query<TCollection>(Expression<Func<TCollection, bool>> selector)
where TCollection : class, IPersistent
{
return Repository.For<TCollection>().Where(selector);
}
(I should note here that my Repository implementation implements IQueryable<T> directly and then delegates the work down to the NH Session.Query<T>())
And the facading works like this:
public virtual IQueryable<Form> Forms
{
get
{
return Query<Form>(x => x.Account == this);
}
}
This defines the list relationship between Account and Form as the inverse of the actual mapped relationship (Form -> Account).
For 'infinite' collections - where there is a potentially unbounded number of objects in the set - this works OK, but it means you can't map the relationship directly in NHibernate and therefore can't use the property directly in NH queries, only indirectly.
What we really need is a replacement for NHibernate's generic bag, list and set implementations that knows how to use the LINQ provider to query into lists directly. One has been proposed as a patch (see https://nhibernate.jira.com/browse/NH-2319). As you can see the patch was not finished or accepted and from what I can see the proposer didn't re-package this as an extension - Diego Mijelshon is a user here on SO so perhaps he'll chime in... I have tested out his proposed code as a POC and it does work as advertised, but obviously it's not tested or guaranteed or necessarily complete, it might have side-effects, and without permission to use or publish it you couldn't use it anyway.
Until and unless the NH team get around to writing / accepting a patch that makes this happen, we'll have to keep resorting to workarounds. NH and DDD just have conflicting views of the world, here.
This is a follow on question to My earlier question on lazy loading properties. Since the application is an enhancement to a production application in a fairly major enterprise, and it currently running using NHib 1.2, upgrading to version 3 is not going to happen, so the suggested answer of using Lazy Properties in 3.0 won't work for me.
To summarize the problem, I have simple database with 2 tables. One has about a dozen simple fields, plus a one to many relation to the second table as a child table. Two of the fields are very large blobs (several megabytes each), and I want to, effectively, lazy load them. This table has about 10 records, and they populate a grid at start up, but access to the large blobs are only needed for whatever row is selected.
The object structure looks something like this:
[Serializable]
[Class(Schema = "dbo", Lazy = false)]
public class DataObject
{
[Id(-2, Name = "Identity", Column="Id")]
[Generator(-1, Class="native")]
public virtual long Identity { get; set;}
[Property]
public string FieldA { get; set;}
[Property]
public byte[] LongBlob {get; set;}
[Property]
public string VeryLongString { get; set;}
[Bag(-2, Cascade=CascadeStyle.All, Lazy= false, Inverse=true)]
[Key(-1, Column="ParentId")]
[OneToMany(0, ClassType=typeof(DataObjectChild))]
public IList<DataObjectChild> ChildObjects { get; set;}
}
Currently, the table is accessed with a simple query:
objectList = (List<DataObject>) Session.CreateQuery("from DataObject").List<DataObject>();
And that takes care of everything, including loading the child objects.
What I would like is a query to do exactly the same thing, except select a list of the properties of the DataObject that includes everything EXCEPT the two blobs. Then I can add custom property Getters that will go out and load those properties when they are accessed.
So far, I have not been successful at doing that.
Things I have tried:
a) constructing an HQL query using a SELECT list.
It looks something like this:
objectList = (List<DataObject>) Session.CreateQuery(
"SELECT new DataObject " +
"(obj.Identity, obj.FieldA) " +
"from DataObject as obj")
That works, though I have to add a constructor to the DataObject that will construct it from fields I select, which is rather cumbersome. But then it doesn't load and expand the list of child objects, and I can't figure out how to do that easily (and don't really want to - I want to tell NHib to do it.)
b) removing the [Property] attribute from the two fields in the object definition. That keeps the NHibernate.Mapping.Attributes from mapping those fields, so they don't get included in the query, but then I have no way to access them from NHib at all, including writing them out when I go to save a new or modified DataObject.
I'm thinking there has to be an easier way. Can somebody point me in the right direction?
Thanks
I think you're on the right path. However, you're going to have to do more manual work since you're using a very old version of NHibernate. I would fetch projections of your entities that contain only the columns you want to eagerly load. Then when the UI requests the large blob objects, you're going to have to write another query to get them and supply them to the UI.
Another option would be to have a second class (e.g. SmallDataObject) with the same mapping (but without the blobs) and use that for the list. When editing a list item you would load the class with the blobs using the id of the selected list item.
In any case, modifying the mapping after the creation of the SessionFactory is not possible, so you cannot get rid of the mapped properties on demand.
I am using Fluent nHibernate for my persistence layer in an ASP.NET MVC application, and I have come across a bit of a quandry.
I have a situation where I need to use an abstraction to store objects into a collection, in this situation, an interface is the most logical choice if you are looking at a pure C# perspective.
Basically, an object (Item) can have Requirements. A requirement can be many things. In a native C# situation, I would merely accomplish this with the following code.
interface IRequirement
{
// methods and properties neccessary for evaluation
}
class Item
{
virtual int Id { get; set; }
virtual IList<IRequirement> Requirements { get; set; }
}
A crude example. This works fine in native C# - however because the objects have to be stored in a database, it becomes a bit more complicated than that. Each object that implements IRequirement could be a completely different kind of object. Since nHibernate (or any other ORM that I have discovered) cannot really understand how to serialize an interface, I cannot think of, for the life of me, how to approach this scenario. I mean, I understand the problem.
This makes no sense to the database/orm. I understand completely why, too.
class SomeKindOfObject
{
virtual int Id { get; set; }
// ... some other methods relative to this base type
}
class OneRequirement : SomeKindOfObject, IRequirement
{
virtual string Name { get; set; }
// some more methods and properties
}
class AnotherKindOfObject
{
virtual int Id { get; set; }
// ... more methods and properties, different from SomeKindOfObject
}
class AnotherRequirement : AnotherKindOfObject, IRequirement
{
// yet more methods and properties relative to AnotherKindOfObject's intentive hierarchy
}
class OneRequirementMap : ClassMap<OneRequirement>
{
// etc
Table("OneRequirement");
}
class AnotherRequirementMap : ClassMap<AnotherRequirement>
{
//
Table("OtherRequirements");
}
class ItemMap : ClassMap<Item>
{
// ... Now we have a problem.
Map( x => x.Requirements ) // does not compute...
// additional mapping
}
So, does anyone have any ideas? I cannot seem to use generics, either, so making a basic Requirement<T> type seems out. I mean the code works and runs, but the ORM cannot grasp it. I realize what I am asking here is probably impossible, but all I can do is ask.
I would also like to add, I do not have much experience with nHibernate, only Fluent nHibernate, but I have been made aware that both communities are very good and so I am tagging this as both. But my mapping at present is 100% 'fluent'.
Edit
I actually discovered Programming to interfaces while mapping with Fluent NHibernate that touches on this a bit, but I'm still not sure it is applicable to my scenario. Any help is appreciated.
UPDATE (02/02/2011)
I'm adding this update in response to some of the answers posted, as my results are ... a little awkward.
Taking the advice, and doing more research, I've designed a basic interface.
interface IRequirement
{
// ... Same as it always was
}
and now I establish my class mapping..
class IRequirementMap : ClassMap<IRequirement>
{
public IRequirementMap()
{
Id( x => x.Id );
UseUnionSubclassForInheritanceMapping();
Table("Requirements");
}
}
And then I map something that implements it. This is where it gets very freaky.
class ObjectThatImplementsRequirementMap : ClassMap<ObjectThatImplementsRequirement>
{
ObjectThatImplementsRequirementMap()
{
Id(x => x.Id); // Yes, I am base-class mapping it.
// other properties
Table("ObjectImplementingRequirement");
}
}
class AnotherObjectThatHasRequirementMap : ClassMap<AnotherObjectThatHasRequirement>
{
AnotherObjectThatHasRequirementMap ()
{
Id(x => x.Id); // Yes, I am base-class mapping it.
// other properties
Table("AnotheObjectImplementingRequirement");
}
}
This is not what people have suggested, but it was my first approach. Though I did it because I got some very freaky results. Results that really make no sense to me.
It Actually Works... Sort Of
Running the following code yields unanticipated results.
// setup ISession
// setup Transaction
var requirements = new <IRequirement>
{
new ObjectThatImplementsRequirement
{
// properties, etc..
},
new AnotherObjectThatHasRequirement
{
// other properties.
}
}
// add to session.
// commit transaction.
// close writing block.
// setup new session
// setup new transaction
var requireables = session.Query<IRequirable>();
foreach(var requireable in requireables)
Console.WriteLine( requireable.Id );
Now things get freaky. I get the results...
1
1
This makes no sense to me. It shouldn't work. I can even query the individual properties of each object, and they have retained their type. Even if I run the insertion, close the application, then run the retrieval (so as to avoid the possibility of caching), they still have the right types. But the following does not work.
class SomethingThatHasRequireables
{
// ...
public virtual IList<IRequirement> Requirements { get; set; }
}
Trying to add to that collection fails (as I expect it to). Here is why I am confused.
If I can add to the generic IList<IRequirement> in my session, why not in an object?
How is nHibernate understanding the difference between two entities with the same Id,
if they are both mapped as the same kind of object, in one scenario, and not the other?
Can someone explain to me what in the world is going on here?
The suggested approach is to use SubclassMap<T>, however the problem with that is the number of identities, and the size of the table. I am concerned about scalability and performance if multiple objects (up to about 8) are referencing identities from one table. Can someone give me some insight on this one specifically?
Take a look at the chapter Inheritance mapping in the reference documentation. In the chapter Limitations you can see what's possible with which mapping strategy.
You've chose one of the "table per concrete class" strategies, as far as I can see. You may need <one-to-many> with inverse=true or <many-to-any> to map it.
If you want to avoid this, you need to map IRequirement as a base class into a table, then it is possible to have foreign keys to that table. Doing so you turn it into a "table per class-hierarchy" or "table per subclass" mapping. This is of course not possible if another base class is already mapped. E.g. SomeKindOfObject.
Edit: some more information about <one-to-many> with inverse=true and <many-to-any>.
When you use <one-to-many>, the foreign key is actually in the requirement tables pointing back to the Item. This works well so far, NH unions all the requirement tables to find all the items in the list. Inverse is required because it forces you to have a reference from the requirement to the Item, which is used by NH to build the foreign key.
<many-to-any> is even more flexible. It stores the list in an additional link table. This table has three columns:
the foreign key to the Item,
the name of the actual requirement type (.NET type or entity name)
and the primary key of the requirement (which can't be a foreign key, because it could point to different tables).
When NH reads this table, it knows from the type information (and the corresponding requirement mapping) in which other tables the requirements are. This is how any-types work.
That it is actually a many-to-many relation shouldn't bother you, it only means that it stores the relation in an additional table which is technically able to link a requirement to more then one item.
Edit 2: freaky results:
You mapped 3 tables: IRequirement, ObjectThatImplementsRequirement, AnotherObjectThatHasRequirement. They are all completely independent. You are still on "table per concrete class with implicit polymorphism". You just added another table with containing IRequirements, which may also result in some ambiguity when NH tries to find the correct table.
Of course you get 1, 1 as result. The have independent tables and therefore independent ids which both start with 1.
The part that works: NHibernate is able to find all the objects implementing an interface in the whole database when you query for it. Try session.CreateQuery("from object") and you get the whole database.
The part that doesn't work: On the other side, you can't get an object just by id and interface or object. So session.Get<object>(1) doesn't work, because there are many objects with id 1. The same problem is with the list. And there are some more problems there, for instance the fact that with implicit polymorphism, there is no foreign key specified which points from every type implementing IRequirement to the Item.
The any types: This is where the any type mapping comes in. Any types are stored with additional type information in the database and that's done by the <many-to-any> mapping which stores the foreign key and type information in an additional table. With this additional type information NH is able to find the table where the record is stored in.
The freaky results: Consider that NH needs to find both ways, from the object to a single table and from the record to a single class. So be careful when mapping both the interface and the concrete classes independently. It could happen that NH uses one or the other table depending on which way you access the data. This may have been the cause or your freaky results.
The other solution: Using any of the other inheritance mapping strategies, you define a single table where NH can start reading and finding the type.
The Id Scope: If you are using Int32 as id, you can create 1 record each second for 68 years until you run out of ids. If this is not enough, just switch to long, you'll get ... probably more then the database is able to store in the next few thousand years...
I am trying to map a domain model in NHibernate. The domain model is implemented with what I think is DDD style. The mapping works mostly but then when I try to use a collection filter on an a collection I get an exception which says: The collection was unreferenced.
I know the problem comes from how I've implemented the collection. My question: Is it possible to use collection filters in nHibernate on collections implemented this way or should I just forget it, i.e. nHibernate cannot work with this.
The code is as follows:
Person
{
IList<Address> _addresses = new List<Address>();
public string FirstName {get; set;}
...
public void addAddress(Address address)
{
// ... do some checks or validation
_addresses.Add(address);
}
public void removeAddress(Address address) {...}
public ReadOnlyCollection<Address> Addresses
{
get { return new ReadOnlyCollection<Address>(_addresses); }
}
}
The main issue is that I don't want to expose the internal addresses collection publicly.
Every other thing works, I use the field.camelcase-underscore access so nHibernate interacts directly with the field. I've been working through the Hibernate in Action book, an now I'm in chapter 7 where it deals with collection filters.
Is there any way around this. I've got it to work by exposing the internal collection like this:
public ReadOnlyCollection<Address> Addresses
{
get { return _addresses; }
}
but I really dont want to do this.
Help would really be appreciated.
Jide
If I recall correctly - NHibernate filter works as additional clause in sql queries to reduce returned rows from db.
My question to You is - why do You need that?
I mean - how much addresses one person might have? 1? 5? 10?
About collection isolation...
I myself just accept it as a sacrifice for NHibernate (just like argument-less ctor's and "virtual`ity") and use exposed IList everywhere (with private setters) just to reduce technical complexity. Their contents surely can be modified from outside, but I just don't do that.
It's more important to keep code easily understandable than making it super safe. Safety will follow.
I want to use some of predefined lookups without roundrips to Database in NHibernate.
Basically I would like to have code like this:
public class Countries
{
static Countries() {
Australia = new Country
{
Id = 14,
Description = "Australia"
}
}
public static Country Austrlia { get; protected set }
}
Then write this code snippets:
address.Country = Countries.Australia;// Snippet1
if (address.Country == Countries.Australia) { // Snippet2
// Do something
}
So I do override Equals, GetHashCode and even overload operators == and != for Country class lookup.
The Snippet1 works ONLY if the Country with given Id has not been loaded into memory.
Otherwise it throws NonUniqueObjectException saying the object with given Id is already in memory.
To workaraound this I have to evict the loaded country and then assign the lookup value.
This feels wrong and I'm not sure what will happen when the query for Country lookup will be executed again.
So the question is: How to maintain static lookup classes in NHibernate?
Thanks,
Dmitriy.
Check out what we call Enumeration classes..
It's basically the flyweight pattern.
You can persist them, using an IUserType implementation, as the value or the display name.
We do crazy stuff with them. Like provide auto-genned lookup tables with foreign keys for DBA folks while keeping all the values in code.
It seems NHibernate cannot handle 2 objects logically the same (but different instances).
So instead of mixing "static lookup" with Database lookups it is better to only use one of them.
In my case - lookups stored in the database. BUT to avoid roundtrip to the database just for the sake of obtaining the object it is only needed to use Load instead of Get:
address.Country = Session.Load<Country>(CountryIds.Australia); // This does not hit DB
address.Country = Session.Get<Country>(CountryIds.Australia); // This DOES hit DB