Client has a Report, Configuration etc properties. A client can have only one of each. Also, each report can belong to only one Client. This is in a way a one-to-one relationship. Therefore my Report table has a foreignkey column clientID. Same for the Configuration and other tables.
Now as per the definition of one-to-one that i read on the nhibernate site, it means that both the primary keys of Report and Client should be the same. Lets just assume that I cannot implement it that way. Hence to simulate the structure that I have in the database, I have the following mappings:
ReportMap
References(x => x.Client, "clientID").Unique().Not.Nullable();
ClientMap
HasOne(x => x.Report).PropertyRef(x => x.Client).LazyLoad().Cascade.SaveUpdate();
Now the problem I am facing is that when I query for a Client, NHibernate is also generating queries to get the Report, Configuration etc... Also, depending on whether I use Criteria or HQL, the generated queries vary wildly.
var client = session.CreateQuery("from Client as c where c.Id = :clientId")
.SetParameter("clientId", 1L)
.UniqueResult<Client>();
generates one query for the client followed by one query for each property that I mapped as HasOne. ie 2 more queries. One for Report and one for Configuration. HQL generates a total of 3 queries.
However, if I use the Load method or Criteria, it generates one query which joins all the concerned tables.
Despite mapping these collections to be lazy loaded, why is NHibernate fetching them? I really only want information from the Client table.
Whats the logical explanation to this?
From the Nhibernate documentaion on fetching strategies, i understand that single associations are lazy proxy fetched. and that the default fetch strategy of select is executed only when the association is accessed. In my case i am not accessing the association. I am simply reading properties that belong to the Client.
All this is so confusing...
Edit1: I have mapped my one to one relation as mentioned in the nhibernate documentation.
http://nhibernate.info/doc/nh/en/index.html#mapping-declaration-onetoone
There are two varieties of one-to-one
association:
* primary key associations
* unique foreign key associations
Alternatively, a foreign key with a unique constraint, from Employee to Person, may be > expressed as:
<many-to-one name="Person" class="Person" column="PERSON_ID" unique="true"/>
And this association may be made bidirectional by adding the following to the Person >mapping:
<one-to-one name="Employee" class="Employee" property-ref="Person"/>
So technically as i understand, i am not doing anything wrong. This scenario is supposed to be supported by nhibernate.
I'm not sure what the problem is with your query, but I suggest you change your mapping. You have is a one-to-many relationship between Client-Report and a business rule that a client can have only one report. You should map it as such. The reports collection can be mapped as a private member and you can expose a Report property on Client to enforce the business rule. I expect that mapping it this way will resolve your query problem.
You could also map it in the other direction witht Client on the many side if that makes more sense.
Related
I'm trying to get a many to many relationship to work using Fluent Nhibernate.
I have a Product and a RelatedProduct Table
Product {Id, Name...}
ProductRelated{Id, ProductId, RelatedProductId, relOrder}
and a Product class
The mapping looks like
HasManyToMany(x => x.RelatedProducts)
.Table("ProductRelated")
.ReadOnly()
.ChildOrderBy("relOrder asc")
.ParentKeyColumn("ProductId")
.ChildKeyColumn("RelatedProductId");
When a query is done for Product and the RelatedProducts are lazy loaded I can see that the sorting is applied correctly using the relOrder on the join table.
Session.Query<Product>()
.FetchMany(p => p.Categories)
.FetchMany(p => p.Departments)
Once I add in eager loading of the related products NHibernate tries to sort by a relOrder column on the product itself instead of on the join table.
Session.Query<Product>()
.FetchMany(p => p.Categories)
.FetchMany(p => p.Departments)
.FetchMany(p => p.RelatedProducts)
Any ideas of whats going on here?
Well to answer your question what's going on here?, I would say, you are using: "not-together fitting features" of NHibernate.
A snippet from documentation 6.6. Sorted Collections:
Setting the order-by attribute tells NHibernate ...
Note: that lookup operations on these collections are very slow if they contain more than a few elements.
Note: that the value of the order-by attribute is an SQL ordering, not a HQL ordering!
So, this could be applied only for "standard" lazy loading, becuase this kind of a feature is applied only on a DB side. It is not managing order in the memory.
And the eager fetching, as the counter-part, is a different way how to generate and issue the SQL Statement to DB.
So, eager and order-by will never work together.
*
My NOTE: I simply have to append this. I can't help myself
I.
The Eager fetching is the feature which should be avoided (I never use it, but it's me). There is a better solution and it is setting the BatchSize(), which will reduce the 1+N into 1+(a few) and will keep all the (lazy) featrues, including order-by. Check these if interested:
NHibernate QueryOver with Fetch resulting multiple sql queries and db hits
Is this the right way to eager load child collections in NHibernate
https://stackoverflow.com/questions/18419988/
BatchSize() is supported for HasManyToMany as well: ToManyBase:
/// <summary>Specify the select batch size </summary>
/// <param name="size">Batch size</param>
public T BatchSize(int size) { ...
II.
The many-to-many mapping, while fancy at first look, is not the way I'd suggest. Try to rethink your model and introduce the first-level-citizen: PairingEntity - for the pairing object. It will then use many-to-one and one-to-many mapping which could give us more... e.g. improved querying like Subqueries... try to check these:
How to create NHibernate HasManyToMany relation
many-to-many with extra columns nhibernate
Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?
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.
i am trying to turn off lazyloading for one-to-many mapping in NHibernate. I have the follow mapping in my entity mapping class. An entity has many addresses, and what I was looking for is one query that basically join the base table to the Addresses table and return me all the result in one request. Instead I see a series of sql query submitted to database for each record in the base table.
HasMany(m => m.Addresses).Not.LazyLoad().Fetch.Join();
i need a way to turn off lazyloading completely.
I would strongly suggest to read this blog post by Ayende: NHibernate is lazy, just live with it.
Using ORM and trying to avoid laziness... won't work. In case of addresses you will lose paging for example.
(while fetching them via join, what happens? If there will be entity with 10
addresses, and you will ask for first 10 records... you will get just
one. And it could be worse if you will ask for 11...)
But what you can use, is the power of NHibernate: 19.1.5. Using batch fetching
HasMany(m => m.Address)
...
.Fetch.Select()
.BatchSize(25)
Now, if you will need 25 records, there will be 2 SELECTs. First for entity, second for all the related Addresses. That's improvement, while all the advantages of ORM remain.
I think that would be anough:
HasMany(m => m.Addresses).Not.LazyLoad();
To get the data with select you should explicitly use "Fetch":
session.QueryOver<Item>()
.Fetch(item => item.Addresses).Eager
.Take(1000)
.TransformUsing(Transformers.DistinctRootEntity)
.List();
I assume you load the base entity with a HQL, Linq or plain SQL query. Those queries ignore the "join" fetch settings in the mapping. You have to either explicitly fetch the Adresses in the query or use Get/Criteria/QueryOver.
Reference documentation: http://nhibernate.info/doc/nh/en/index.html#performance-fetching-custom
I am just starting out with ADO.net Entity Framework I have mapped two tables together and receive the following error:
Error 1 Error 11010: Association End 'OperatorAccess' is not mapped. E:\Visual Studio\projects\Brandi II\Brandi II\Hospitals.edmx 390 11 Brandi II
Not sure what it is I am doing wrong.
I believe I can add some more clarity to the issue (learning as I go):
When I look at the Mapping details and look at the association, the column for operatoraccess table (from above) is blank and the drop down only includes field from the linked table.
The Entity Framework designer is terrible - I've had the same problem many times (and your problem too, Craig):
This happens when you have a many-to-one association which is improperly setup. They could very easily fix the designer to make this process simple; but instead, we have to put up with this crap.
To fix:
Click on the association, and go to the mapping details view.
Under association, click on Maps to <tablename>. Choose the table(s) which make up the many side of the relationship (ie. the table(s) which make up the *-side of the association in the designer)
Under Column, choose the table-columns which map to each entity-side Property. You get this error when one of those entries are blank.
I had the exact same problem and this is what I did to fix it.
Make sure you have an Entity Key set in your designer on the tables your making an association with. Also check that StoreGeneratedPattern is set to Identity for that Entity Key.
There's not a lot of information in your question, but, generally speaking, this means that there is an incompletely defined association. It could be that you have tried to map one table with a foreign key to another table, but have not mapped that other table. You can also get this error when you try to do table per type inheritance without carefully following the steps for implementing that feature.
Not sure of the answer, but I've just posted a similar question, which may at least help clarify the issue you are experiencing.
Defining an Entity Framework 1:1 association
I had to go back into the database itself and clarify the foreign key relationship
I had this problem in the case where I was creating both many to 0..1 and 0..1 to 0..1 associations. One entity needed associations to multiple tables, and that entity did not have foreign keys defined for those tables.
I had to do the table mappings step that is given in the accepted answer, but note that it wasn't only for many to many associations; it applied to all the types of associations I added for this entity.
In the Mapping Details view, I had to select the entity with the non-foreign key ID columns to the various tables. This is not always the "many" side of the relationship. Only there was I able to map the related entity property to the appropriate property in the original entity. Selecting the "destination" entity would not allow me to select the properties that I needed to, and the error would still exist.
So in short, I had to map using the table related to the entity that had the "non-foreign key" ID fields corresponding to the various entities' (and their tables') primary keys that I needed to associate.
Entity A
various other properties...
Id
ContactId
OrderId
etc.
Contact entity
Id
FirstName
LastName
etc.
In the mapping details, I selected Entity A's table. It then showed both ends of the association. I mapped its Entity A's Id property to its table's actual ID column (they had different names). I then mapped the Contact entity's Id field to the ContactId field on the A entity.
Simply select the many relationship table (*) from the Association>Edit Mapping & select the appropriate relationship