I generated a Members table and a MembersType table which has a primary key which links to the Type foreign key in the Members table. The MembersType table is literally just 3 records, so that each Member can be of MemberType 1, 2 or 3.
Now the problem is that when Entity Framework generates the data layer and objects for the Members object, it creates a MemberType object in the Members object, but all I want to be able to do when setting it is:
Members.MemberType = 1;
but because of the above, I have to do this:
MemberTypes = db.MemberTypes.Where(x => x.MemberTypeId == 1).AsQueryable().First()
Is there anyway to stop it from generating an object on foreign keys so I can just set it as an int? Surely this is more quicker and resource efficient than querying the type table everytime too.
You have encountered one of everyone's least favorite features of EFv1. The problem is that everything is an Entity, so you can't get to foreign key values as primitives.
Your code sample shows how it has to be done in EFv1. The best you can do is cache those enum values up front so you don't have to keep getting them from the context. EFv4 does away with this restriction with "FK Properties," which is just a fancy way of saying raw foreign keys you can set directly.
Related
I'm working with a rather large EF core model that has all tables and relationships defined in code that wind up attached to a context object. If I have a primary key, is there a clean/efficient way to get all the dependent columns in the model?
As an example of what I'm working with, this snippet works to find a primary key's name for a given table (the table is known by its type T which is a parameter to the function in which this code lives):
var keyName = context.Model.FindEntityType(typeof (T)).FindPrimaryKey().Properties.Select(x => x.Name).Single();
I found the answer while poking through the properties of they key object. Instead of getting the name, get the object itself, then call these two other methods to get the foreign keys that reference the primary key:
var key = context.Model.FindEntityType(typeof (T)).FindPrimaryKey().Properties.FirstOrDefault();
var foreignkeys = key.GetContainingPrimaryKey().GetReferencingForeignKeys();
Just starting out with Entity Framework (Code First) and I have to say I am having a lot of problems with it when loading SQL data that is fairly complex. For example, let's say I have the following tables which stores which animals belongs to which regions in the world and the animal are also categorized.
Table: Region
Id: integer
Name string
Table AnimalCategory
Id integer
Name: string
RegionId: integer -- Refers back Region
Table Animal
Id integer
AnimalCategoryId integer -- Refers back AnimalCategory
Let's say I want to create a query with Entity Framework that would load all Animals for a specific region. The easiest thing to do is to create 3 Entities Region, AnimalCategory, and Animal and use LINQ to load the data.
But let's say I am not interested in loading any AnimalCategory information and define an Entity class just to represent AnimalCategory so that I can do the JOIN. How can I do this with Entity Framework? Even with many of its Mapping functions I still don't think this is possible.
In non Entity Framework solutions this is easy to accomplish by using INNER JOINs in SPs or inline SQL. So what are my options in Entity Framework? Shall I pollute my data model with these useless tables just so I can do a JOIN?
It's a matter of choice I guess. EF choose to support many-to-many associations with transparent junction tables, i.e. where junction tables only have two foreign keys to the associated entities. They simply didn't choose to support this far less common "skipping one-to-many-to-many" scenario in a similar manner.
And I can imagine why.
To start with, in a many-to-many association, the junction table is nothing but that: a junction, an association. However, in a chain of one-to-many (or many-to-one) associations it would be exceptional for any of the involved tables to be just an association. In your example...
Animal → AnimalCategory → Region
...AnimalCategory would only have a primary key (Id) and a foreign key (RegionId). That would be useless though: Animal might just as well have a RegionId itself. There's no reason to support a data model that doesn't make sense.
What you're after though, is a model in which the table in the middle does carry information (AnimalCategory.Name), but where you'd like to map it as a transparent junction table, because a particular class model doesn't need this information.
Your focus seems to be on reading data. But EF has to support all CRUD actions. The problem here would be: how to deal with inserts? Suppose Name is a required field. There would be no way to supply its value.
Another problem would be that a statement like...
region.Animals.Add(animal);
...could mean two things:
add an Animal and a new AnimalCategory, the latter referring to the Region.
Add an Animal referring to an existing AnimalCategory - without being able to choose which one.
EF wouldn't want to choose for some default behavior. You'd have to make the choice yourself, so you can't do without access to AnimalCategory.
I am using EF 4.0 , i have one problem
Table structure in DB is:
Table: Setting--->
Name (PK)
GroupBy
DataType
Table: UserSetting-->
SettingName(PK)(FK)
UserName(PK)(FK)
Value
Table: WorkstationSetting-->
SettingName(PK)(FK)
WorkstationName(PK)(FK)
Value
Now i want to make use of inheritance, because WorkstationSetting and UserSetting inherits settings so any suggestion how to achieve inheritance, i tried but i got error like
"Error 39 Error 3003: Problem in mapping fragments starting at line 1621:All the key properties (Settings.Name) of the EntitySet Settings must be mapped to all the key properties (WorkstationSetting.SettingName, WorkstationSetting.WorkstationName) of table WorkstationSetting.
I see you have in UserSetting and WorkstationSetting a composite PK.
If UserSetting and WorkstationSetting are derived from Setting, they should have Name as PK.
Another comment; in general, it's not recommended to use a name or something "meaningful" as PK since it is less scalable and might cause limitations (i.e. max index size). Use instead an int or uniqueidentifier.
I recommend you to introduce a new field which is SettingId which should be added to all three tables. In EF designer, just add the Inheritance.
Look into table per type inheritance. For example look here. It should help you get started. The idea is that you have a table for each concrete type (as you have) and you map it to an object hierarchy.
Maybe your problem is with the keys. How is your mapping defined? Are the associations between the tables defined in the DB?
In trying to normalize a database schema and mapping it in Entity Framework, I've found that there might end up being a bunch of lookup tables. They would end up only containing key and value pairs. I'd like to consolidate them into one table that basically has two columns "Key" and "Value". For example, I'd like to be able to get Addresses.AddressType and Person.Gender to both point to the same table, but ensure that the navigation properties only return the rows applicable to the appropriate entity.
EDIT: Oops. I just realized that I left this paragraph out:
It seems like a TPH type of problem, but all of the reading I've done indicates that you start with fields in the parent entity and migrate fields over to the inherited children. I don't have any fields to move here because there would generally only be two.
There are a lot of domain-specific key-value pairs need to be represented. Some of them will change from time to time, others will not. Rather than pick and choose I want to just make everything editable. Due to the number of these kinds of properties that are going to be used, I'd rather not have to maintain a list enums that require a recompile, or end up with lots of lookup tables. So, I thought that this might be a solution.
Is there a way to represent this kind of structure in EF4? Or, am I barking up the wrong tree?
EDIT: I guess another option would be to build the table structure I want at the database level and then write views on top of that and surface those as EF entities. It just means any maintenance needs to be done at multiple levels. Does that sound more, or less desireable than a pure EF solution?
Table per hiearchy demands that you have one parent entity which is used as base class for child entities. All entities are mapped to the same table and there is special discriminator column to differ type of entity stored in database record. You can generally use it even if your child entities do not define any new properties. You will also have to define primary key for your table otherwise it will be handled as readonly entity in EF. So your table can look like:
CREATE TABLE KeyValuePairs
(
Id INT NOT NULL IDENTITY(1,1),
Key VARCHAR(50) NOT NULL,
Value NVARCHAR(255) NOT NULL,
Discriminator VARCHAR(10) NOT NULL,
Timestamp Timestamp NOT NULL
)
You will define your top level KeyValuePair entity with properties Id, Key, Value and Timestamp (set as concurrency mode fixed). Discriminator column will be used for inheritance mapping.
Be aware that EF mapping is static. If you define AddressType and Gender entities you will be able to use them but you will not be able to dynamically define new type like PhoneType. This will always require modifying your EF model, recompiling and redeploying your application.
From OOP perspective it would be nicer to not model this as object hiearchy and instead use conditional mapping of multiple unrelated entities to the same table. Unfortunatelly even EF supports conditional mapping I have never been able to map two entities to the same table yet.
I'm getting more and more frustrated with EF...
I have a table called ExtendedField with:
Record
DocRef
DocType
Name
Record is the primary key
DocRef and DocType are foreign keys used to identify which Ticket they belong to
Name is the key used by the "definition" table to define what the field actually is
So basically I need 2 associations:
One between Ticket and ExtendedField
on ExtendedField.DocRef=ticket.record
and
ExtendedField.docType=HeaderDocType
One between Definition on
ExtendedField.Name=Definition.FieldName
Then I still need Record to be the primary key so I can directly access the fields.
As near as I can tell this is impossible to do in Entity Framework. For every association all the keys need to be mapped together, whereas I need two keys for one association, 1 key for another one and the actual primary key wouldn't be used in any associations.
It doesn't appear that you can define an association between fields that aren't entity keys either.
So is there any way to do this? Am I missing something?
It's a v1, bro. I myself have had some major pain with mapping of key constraints in EF. I hear that better things are coming in v2.