I have a Subscriber object that contains a list of Provider objects. Providers can belong to many Subscribers, hence the many-to-many relationship. This is fine, except that the Provider needs to define a Status property but this cannot be stored in the Provider table, as the same provider could have a different Status for different Subscribers, so I am storing the Status in the many-to-many table. At the moment I have a basic many-to-many mapping:
HasManyToMany(s => s.Providers)
.Table("SubscriberProviders")
.ParentKeyColumn("SubscriberID")
.ChildKeyColumn("ProviderID");
How can I set the Status property, of the Provider, within the many-to-many mapping?
Many thanks
A many-to-many mapping can't have properties of its own, so you have to map the join table into an artificial ProviderSubscriber entity, which will be one-to-many from the Provider.
For a full example of a workaround, see Many-to-many relationships with properties
You'll have to map the cross-reference table (which NH currently generates for you), and change the mapping between Providers and Subscribers to instead be a HasMany() on either side referencing the cross-reference table.
Related
I have a design problem with regards to Entity Framework model relationship
I have this model in the edmx
Business Rule:
A Participant can have multiple Roles so I create a relationship table ParticipantRoles that has 1-to-Many relationship on the Participant and the Role table
The Problem:
In order to get the Participant's Role value, I have to drill down through Participant->ParticipantRole->Role (see JSON output below)
The Question:
In EF, how to design the table relationship to bypass the ParticipantsRole table. I want to access the Role in something like this Particant.Role and not Participant.ParticipantsRole.Role
You say A Participant can have multiple Roles. And of course, a Role can have multiple Participants. So basically this is a many-to-many association.
Entity Framework will only map pure many-to-many associations (without connecting class) when the junction table only has two foreign keys. In your case, if the table ParticipantsRole only would have had a primary key consisting of ParticipantId and RoleId at the time of generating the model the class ParticipantsRole would not have been created. You would have had Participant.Roles and Role.Participants as navigation properties.
However, the model has been generated with ParticipantsRole and you want to get rid of it. (Or not, I'll get back to that).
This is what you can do:
Remove ParticipantRoles from the class diagram.
Modify the database table ParticipantRoles so it only has the two FK columns, that both form the primary key.
Update the model from the database and select ParticipantsRole in the Add tab.
This should give you a model with a pure many-to-many association.
However, think twice before you do this. M2m associations have a way of evolving into 1-m-1association (as you've got now). The reason is that sooner or later the need is felt to record data about the association, so the junction table must have more fields and stops being a pure junction table. In your case I can imagine that one day participant's roles must have a fixed order, or one marked as default. It can be a major overhaul to change a m2m association into 1-m-1 in a production environment. - Something to consider...
I have a mapping in FluentNHibernate for a HasMany relationship and I'd like to specify a Table on it to override the default Table that nHibernate will look in to find those objects that I have many of. Does that make sense?
So lets say I have a table for Invoices and a table for InvoiceItems and lets say I have table called InvoiceItemsTwo.
I have a class for Invoice and a Class for InvoiceItems as well, and their mappings are pretty straight forward. I'd like to specify in my mapping for Invoice, that it should look for it's items in InvoiceItemsTwo instead of the default InvoiceItems.
So my mapping of that relationship looks like this
HasMany(c => c.InvoiceItems).Cascade.SaveUpdate().Table("InvoiceItemsTwo");
But this doesn't work. I keep getting an error from my website at runtime that says Invalid object name 'InvoiceItems'.
Why is it ignoring the fact that I am explicitly specifying the Table in my mapping on the relationship?
I tried dumping the mapping at run time and it's being setup something like this
<bag cascade="save-update" table="InvoiceItemsTwo">
Any ideas?
The table attribute applies only to many-to-many relationships, not one-to-many.
you can't specify a different table in your mapping class. Fluent NHibernate uses the class mapped on the property list (InvoiceItems).
If yoy want to use another class to map your details table you must create a InvoceItemsTwo class and map it in your master table class.
You could map the list as composite-element instead of a one-to-many relation and then map it to another table. But it is not a good idea. Consider that NH needs to know where to store an object which is in memory. So it may happen that the object is stored in the wrong table.
Either store all the InvoiceItems in separate tables using composite-element instead of one-to-many and components instead of many-to-one (however this is called in Fluent).
Or store all the InvoiceItems in the same table and use regular references.
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.
I'm using an ORM (SQLAlchemy, but my question is quite implementation-agnostic) to model a many-to-many relationship between a parent class and its children.. I was wondering, what would be a simple way to express the concept "one of the children is the default/main one"?
For example, I'd need to persist the following:
This Person instance has Address X and Y, the main one is Y.
I saw this implemented using a "middle" class like "PersonAddressRelation" that would contain "Person", "Address" and the "main" flag, but I think it looks a bit cumbersome.. Is there a better way?
The simplest way would be to have a join table, PersonAddressRelation, and also a DefaultAddress column on the Person table that keys to the Address table.
A couple of remarks.
M:N relationships don't specify 'parent' and 'child', as there's no parent nor a child: there are simply two entities having an m:n relationship via a 3rd entity (the intermediate entity).
'Address' is in general not a valid entity type, as semantically it has no identity, similar to a 'name' has no identity (first name, last name). You'll see this when you look at re-using an entity instance of type Address: you won't do that in general. (though you will re-use a Customer entity instance for example, when the customer has multiple orders)
You want to specify an attribute on the M:N relationship (default), as it belongs there. This means that the relationship itself forms an entity (which is the intermediate entity, often it has just two FK fields forming the PK). This is called an 'objectified relationship', as the relationship itself is seen as an entity. Other examples of this are Employee m:n Department and you want to specify the StartDate an employee started for a department the employee works for.
So in general: create the intermediate entity, as it in general should be there, and add the attribute there. In this particular case with Address, be really sure you are re-using Address instances among related entities (Person). If not, merge Address with Person OR if a person can have multiple addresses, create a simple 1:n relationship between Person - Address, to normalize it out, though don't be afraid to merge address data into the entity it is related to, as often address data is really not re-used (so your m:n relationship is really not there: there's no Address instance which is related to multiple person instances.
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