I am stuck here.
Is it possible to map data from 2 different tables to 1 entity in Entity Framework 4.
I have a bunch of employees in one table, and in the other I have som project information.
I would like to combine these 2 tables in one Entity, and keep the tracking features etc., is that possible?
I do not want to use a function import, but do it solely through the Entity Model.
Can anyone help - when I try to do it, i get the following error all the time:
Error 3024: Problem in mapping fragments starting at line 2354:Must specify mapping for all key properties (MyProjectTable.PSInitials, MyProjectTable.ProjectID) of the EntitySet MyProjectTable.
Both key are mapped to their respective tables.
The new Entity are made with MyProjectTable as the basetable.
The relation between the 2 tables is a 1-*
Hope you can help.
/Christian
You cannot map two tables with a one-to-many relationship to one entity. If you don't want projecting the results into one object in code, consider creating a view and mapping it instead.
According to http://msdn.microsoft.com/en-us/library/bb896233.aspx
You should only map an entity type to
multiple tables if the following
conditions are true:
The tables to which you are mapping share a common key.
The entity type that is being mapped has entries in each
underlying table. In other words,
the entity type represents data
that has a one-to-one correspondence between the two
tables; the entity type represents an
inner join of the two tables.
The reasons for doing this are quite straightforward - for example, a table of data points that all have one of five 'types'. Obviously the 'type' will be a separate table for the sake of normalisation, but from an application point of view (working with the data) it makes more sense to have all properties in a single entity.
So we can't do this with Entity Framework - a supposed Object-Relational-Mapper. What, then, is the point of using such a framework?
Related
Here's my question. I have 2 models (Person, Event) and with EF and modelbuilder I generate a booking table (with IdPerson and IdEvent as properties).
So in my DB it's correct, I have 3 tables (Person, Event and Booking) with many to many relationship. But I have only 2 models in Visual Studio (Booking doesn't exist because of the self-generated table).
With my Controller I want to write an action for the Person to suscribe to an event and I have to write on my table Booking on the DB but it doesn't exist as a model so I can't do that .
How should I proceede?
Should I create a Booking model and delete my modelbuilder?
When you are using ORMs like EF, you can sit back and let the ORM manage these middle tables.
You can use
person.Events.Add(event)
or
event.People.Add(event)
and EF handles all and inserts a row with personId and eventId in that table.
Here you can find a complete sample:
http://blogs.msdn.com/b/wriju/archive/2011/05/14/code-first-ef-4-1-building-many-to-many-relationship.aspx
I assume this is a model first approach.
The reason for having only 2 objects is that, by default, EF does not create objects for joint tables. What it does create is Navigation Property (Entity Framework - Navigation Property Basics). In one-to-many scenario, a navigation property inside a parent object contains a collection of entities in a foreign / child table. In many-to-many scenario, navigation properties of each entities will simply contain collections of its other entities.
I am extracting data from a set of JSON files into an MS T-SQL database using Entity Framework.
There are a bunch of sub-collections in the JSON files (counted 20 so far) that all follow the pattern "CollectionName":{"Code":"SomeCode","Description":"Some Description"}.
For example: "Country":{"Code":"GB","Description":"Great Britain"} or "Language":{"Code":"FR","Description":"French"}
The code I am working with uses this pattern: an Entity is created called CollectionName, which maps to a table with PK, Code & Description columns and then another Entity called SourceCollection (ex: PersonLanguage) which maps to a Bridge Table having the Source PK & CollectionName PK for each. When you have a lot of these little sub-collections, that is a lot of tables.
As a T-SQL programmer, I have solved similar problems in the past by creating a 'catchall table' that has a PK, a CollectionName Column, and then a Code & Description column as above. So all these little collections reside in a single table, with a foreign key pointer in the source table.
I cannot find any description of how to implement this in Entity Framework, can anyone point me in the right direction with either a link or some sample code?
The pattern you're describing is sometimes called a "common lookup table" and is generally considered an anti-pattern for reasons of referentially integrity and constraints.
Merits of the design decision aside, you have two options:
A) Create a new EF entity with properties for Id, CollectionName, Code and Description and map your existing classes and data to that entity for CRUD operations via some pattern such as Repository.
B) Use EF type inheritance with table-per-hierarchy mapping and allow EF to map multiple entities to the same table. The abstract parent type would have Id, Code and Description properties. EF will automatically create a discriminator column that serves the same purpose as CollectionName.
If I have for example two entities, lets say Customers and Staff with no relation between them, is it possible to create a third entity which doesn't have a corresponding table in the database which takes some information from the first and the second entity and also one or two additional columns (for example computed columns)?
You can join entities through this: http://blogs.msdn.com/b/simonince/archive/2009/03/23/mapping-two-tables-to-one-entity-in-the-entity-framework.aspx
But I do believe they have to be related; otherwise, how would it know what information was correctly tied to each other? The only thing it could do is query all rows of one entity and match them up with all rows of the other, which is probably not what's desired.
Such a task: we have 2 tables in our L2S Entity classes. It needs to manage with current fields of current tables by numbering em somehow.
Exact question is How can I point to the exact field of exact table without using entity relation names? Such as TmpLinqTable[2] instead of TmpLinqTable.TableField.
Moreover if it can be managed by ids of the entity, not the table.
So my understanding of what you are trying to do is to log changes that happen to your entites. Is that correct? You might want to look into the GetModifedMembers method on the Table class. Here's an interesting link...
http://geekswithblogs.net/steveclements/archive/2008/04/15/linq-to-sql-property-changed--changing-logging.aspx
I am trying to come up with a database design that would work with Entity Framework 4 Code First. Actually, I have no experience yet of EF4 Code First but as I understand it, if I write the code, it will create the database and tables.
The issue is this. There are various types of auctions, they all have some common fields and some specific ones. In the code I envisage having a base abstract class called Auction and subclasses like LowestUniqueBidAuction and EnglishForwardAuction etc.
Nothing surprising there. The problem is that I imagine the database structure to mimic this. I imagine an Auction table and a LowestUniqueBidAuction table and a EnglishForwardAuction table. In the Auction table I imagine a foreign key into one of these two tables for each row depending on the type of auction that that row is. I also imagine another column in the Auction table with the name of the derived auction table (such as EnglishForwardAuction).
The problem is that whenever I've ever created a foreign key I've had to specify the name of the foreign table into which the key points (which makes sense). In this case, however, there is one of many tables that the key could point. So there are many issues here. Firstly, I could simply not use a foreign key and just use an ordinary field, but then the database will not be able to maintain data consistency for me. The second issue is how will EF Code First handle this? In other words, how will it know that if I ask for all EnglishForwardAuction rows from the Auction table that it should look at the column with the table name and then join on the EnglishForwardAuction table to get the extra fields?
Has anyone ever faced similar issues?
Thanks,
Sachin
This problem is solvable in Entity Framework in a number of ways - read up on how EF handles inheritance and what strategies are available.
There are basically three strategies how to handle this:
(1) Table per Hierarchy
You have only one single table, that represents all possible sub classes. Of course, this means, several rows (that only exist in a given subclass) must be nullable, since they don't show up / don't exist in super classes or other subclasses.
(2) Table per Type
Each subclass gets its own table, and by default, the sub-types table shares the PK with the base classes' table - e.g. PK = 1 in Auction will also be PK = 1 in EnglishForwardAuction. So your subclass tables reference the base table - not the other way around.
(3) Table per Concrete Type
Each concrete subclass (your separate auction types) gets its own table, but that table contains everything - all the columns, from that specific type, but also its base type.
Read more here:
Inheritance in the Entity Framework
Inheritance and Associations with Entity Framework Part 1
Entity Framework Modeling: Table Per Hierarchy Inheritance
Entity Framework Modeling: Table Per Type Inheritance
Searching for Entity Framework Inheritance and/or one of these strategies will reveal a lot more hits, too - that topic is very well covered and discussed on the interwebs! :-)