If I have a database with courses and course templates, and company course templates, what is the best way to store them in a SQL database? Course will probably have some other metadata about score, time-taken, and perhaps some other info.
Table Per Hierarchy
// Classes
public class CourseTemplate { }
public class CompanyCourseTemplate { }
public class Course { }
Table Per Type
public class CourseTemplate { }
public class CompanyCourseTemplate : CourseTemplate { }
public class Course : CompanyCourseTemplate { }
// Also for Course what if there are Course that belong to
// either CompanyCourseTemplate or else CourseTemplate
Hybrid Using Both - TPH
public class CourseTemplate { }
public class CompanyCourseTemplate : CourseTemplate { }
And - TPT
public class Course { }
It seems like the querying always gets messy when you need to differentiate the types within a table. As well, if you create a table per type the database gets cluttered with similar named tables as well. I am leaning towards the hybrid since it seems to be more natural with the types.
However, let's say we are storing a reference to the templates and we are using table per type what is the best way to manage this relationship?
In the exam table I could have two foreign keys to the template tables.
public class ExamTemplate { }
public class CompanyExamTempalte { }
public class Exam {
public Int32 ? ExamTemplateId { get;set; }
public Int32 ? CompanyExamTemplateId { get;set; }
The problem with this is that my code gets really ugly when I have to get an template from an exam template reference. I haven't found the best way to really take care of this problem. What is the best way to inherit from one of two templates. I am thinking that a hierarchy needs to be created where all exam instances must have a company exam template as well as a master template and the exam can hold references to both. The change would look like this.
public class Exam {
public Int32 CompanyExamTemplateId { get;set; }
public CompanyExamTemplate { get; set; }
public Int32 ExamTemplateId { get; set; }
public ExamTemplate { get; set; }
}
NOTE the nullables are gone since the hierarchy is always required.
Seems like you were initially leaning toward a one-to-many parent-child table structure something like this:
tblCourseTemplate:
CourseTemplateID
CourseTemplateName
CourseTemplateIsCompanyTemplate
CourseTemplateCompanyNameIfApplies
tblCourse:
CourseID
CourseTemplateID (i.e. parent record)
CourseName
CourseScore
CourseDateTaken
CourseTimeTaken
But then you said that a course could have more than one "parent" record so perhaps use an intermediate table to handle a many-to-many relationship:
tblCourseTemplate:
CourseTemplateID
CourseTemplateName
CourseTemplateIsCompanyTemplate
CourseTemplateCompanyNameIfApplies
tblCourseTemplateAndCourse:
CourseTemplateAndCourseID
CourseTemplateID
CourseID
tblCourse:
CourseID
CourseName
CourseScore
CourseDateTaken
CourseTimeTaken
I am trying to map a vertical inheritance between a base class and derived class (obviously). I am using code-only and the FluentAPI approach for which I have found almost ZERO documentation. I have found a couple of docs on vertical inheritance and code-only but very few on managing the discriminator column/value.
So I have been trying to extrapolate how to do it from a combination of this blog post and some documentation on implementing vertical inheritance using code-only. All to no avail.
You will see that I have a "Deliverables" base table and "PrintDeliverables" derives from that. There will be other derivatives coming down the road. But I figured I would start with one first.
Anyway, I naturally have models that map to the tables.
public class PrintDeliverable : BDeliverableBase
{
public String PaperItemNumber { get; set; }
public String PrinterModel { get; set; }
public Boolean? ColorOption { get; set; }
public String ProductCode { get; set; }
}
public class BDeliverableBase : BModelBase, IDeliverable, ISingleID
{
public Int64 ID { get; set; }
public String Label { get; set; }
public String Description { get; set; }
public IList<DeliverableAttribute> Attributes { get; set; }
public Int64 TypeID { get; set; }
public DeliverableType Type { get; set; }
}
public class DeliverableType : BModelBase, ISingleID
{
public Int64 ID { get; set; }
public String Label { get; set; }
public String Description { get; set; }
public IList<BDeliverableBase> Deliverables { get; set; }
}
I have standard mapping which maps the fields and types, sizes, etc. When I run it with no further additions I get the Error ...
Invalid Column name voa_class
My research uncovered that the ORM is attempting to update a "discriminator" column with a value that will tie the base table and table with the derived data together. I learned that I can change the name of the column it uses, which I did in the BASE CLASS mapping (BDeliverableBase). I changed it to use the "DeliverableTypeId" column since the DeliverableType indicates which TYPE of deliverable it is. Since each TYPE will have it's own derivative table this would be an appropriate value to associate which derivative table to use.
MappingConfiguration<BDeliverableBase> map = new MappingConfiguration<BDeliverableBase>();
map.HasDiscriminator().ToColumn("DeliverableTypeId");
It appears to like this better but it wants to insert this crazy number (ex// 612274703-854) into the DeliverableTypeId column which, of course, being a foreign key to the DeliverableTypes table is not allowed.
Insert of '612274703-' failed: Telerik.OpenAccess.RT.sql.SQLException: The INSERT statement conflicted with the FOREIGN KEY constraint "FK_DeliverableType". The conflict occurred in database "DB1", table "dbo.DeliverableTypes", column 'DeliverableTypeId'
I learned that OpenAccess/DataAccess generates a hash value to insert into the discriminator column. I do not want this, in fact I know that the value must be one of the IDs available in the DeliverableType. So I read in one of the docs that you could define what value to assign to the discriminator. The example applied a hard-coded value to the base class (dog and cat derived from animal) ...
animal.HasDiscriminatorValue("23");
This presented one problem ... I do not have a single value I can hard-code. It could one of MANY values present in the DeliverableTypes table. However, for the sake of proving out the concept I hard-coded the value of an existing record
MappingConfiguration<BDeliverableBase> map = new MappingConfiguration<BDeliverableBase>();
map.HasDiscriminator().ToColumn("DeliverableTypeId");
map.HasDiscriminatorValue("819");
I continued to get the identical error from before. So it doesn't appear that it was applying my hard-coded value. So ... I thought, while hard-coding the value is a little hacky it would make more sense to define that in the mapping for the derived class. That would resolve my hard-coded issue since ALL instances of that derived class WOULD indeed be of the same DeliverableTypeId. So I tried ...
MappingConfiguration<BDeliverableBase> map = new MappingConfiguration<BDeliverableBase>();
map.HasDiscriminator().ToColumn("DeliverableTypeId");
MappingConfiguration<PrintDeliverable> map = new MappingConfiguration<PrintDeliverable>();
map.HasDiscriminatorValue("819");
This resulted in the Error
Insert of '612274703-857' failed: Telerik.OpenAccess.RT.sql.SQLException: String or binary data would be truncated.
So I got a different error but still the same poblem. This type it was trying to stuff the ORM generated discriminator value (instead of my 819) into what I am assuming is my defined discriminator column (DeliverableTypeId), although the different error makes me suspicious that it was targeting a different column.(?)
In an effort to not drag this out too long I have tried several combinations of where to these "HasDiscriminator" and "HasDiscriminatorValue" assignments go but always end up with one or the other of these errors. So the question is ...
How, using code-only, do I map Vertical Inheritance using multiple, existing "type" values?
I have entity like below
public abstract class MyBaseClass
{
public int Id { get; set; }
}
public class MyConcrete : MyBaseClass
{
public int TemplateName { get; set; }
public int Total { get; set; }
}
public class MyOtherConcrete : MyBaseClass
{
public int TemplateName { get; set; }
public int MyProperty { get; set; }
public string OtherProperty { get; set; }
}
using default initialization, EF will make table with columns like bellow:
Id
TemplateName
TemplateName1 // <-- this is the problem
Total
MyPorperty
MyOtherProperty
now my question how to configure EF so all the TemplateName property on derived class automatically mapped into TemplateName column without making another column. is it possible to configure it on OnModelCreating method?
EDIT
actually above was simplified version of my problem. i have 10 more entities some property might duplicated everywhere and i don't want to add any abstraction layers.
i have tried manually map the column on the OnModelCreating but having "Each property name in a type must be unique. Property name 'TemplateName' was already defined" exception any idea?
EDIT 2
so, i found here, that said it is impossible to do such thing like above in EF, it is weird for me..
Move TemplateName into MyBaseClass to avoid this problem.
If necessary, you can use intermediate base classes to hold properties shared by only a subset of your classes.
After searching through the net, so far that was not possible to do that. since i realize that my inheritance tree is wrong.
so in my case, i should change my code to match the EF requirement, it is sound weird.. because in many case ENTITY is a NO NO to change, we usually create an entity that used in multiple project. event we found our entity wrong we won't update it because updating it will require massive change on the other projects.
so far, i think there is no exact answer for my question. will update it soon after EF support it.
I'm having problems setting up an Entity Framework 4 model.
A Contact object is exposed in the database as an updateable view. Also due to the history of the database, this Contact view has two different keys, one from a legacy system. So some other tables reference a contact with a 'ContactID' while other older tables reference it with a 'LegacyContactID'.
Since this is a view, there are no foreign keys in the database, and I'm trying to manually add associations in the designer. But the fluent associations don't seem to provide a way of specifying which field is referenced.
How do I build this model?
public class vwContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class LegacyObject
{
public virtual vwContact Contact { get; set; }
public string ContactId { get; set; } //references vwContact.LegacyKeyField
}
ModelCreatingFunction(modelBuilder)
{
// can't set both of these, right?
modelBuilder.Entity<vwContact>().HasKey(x => x.KeyField);
modelBuilder.Entity<vwContact>().HasKey(x => x.LegacyKeyField);
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).???
//is there some way to say which key field this reference is referencing?
}
EDIT 2: "New things have come to light, man" - His Dudeness
After a but more experimentation and news, I found using a base class and child classes with different keys will not work by itself. With code first especially, base entities must define a key if they are not explicitly mapped to tables.
I left the suggested code below because I still recommend using the base class for your C# manageability, but I below the code I have updated my answer and provided other workaround options.
Unfortunately, the truth revealed is that you cannot accomplish what you seek without altering SQL due to limitations on EF 4.1+ code first.
Base Contact Class
public abstract class BaseContact
{
// Include all properties here except for the keys
// public string Name { get; set; }
}
Entity Classes
Set this up via the fluent API if you like, but for easy illustration I've used the data annotations
public class Contact : BaseContact
{
[Key]
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class LegacyContact : BaseContact
{
public int KeyField { get; set; }
[Key]
public string LegacyKeyField { get; set; }
}
Using the Entities
Classes that reference or manipulate the contact objects should reference the base class much like an interface:
public class SomeCustomObject
{
public BaseContact Contact { get; set; }
}
If later you need to programmatically determine what type you are working with use typeof() and manipulate the entity accordingly.
var co = new SomeCustomObject(); // assume its loaded with data
if(co.Contact == typeof(LegacyContact)
// manipulate accordingly.
New Options & Workarounds
As I suggested in comment before, you won't be able to map them to a single view/table anyway so you have a couple options:
a. map your objects to their underlying tables and alter your "get/read" methods on repositories and service classes pull from the joined view -or-
b. create a second view and map each object to their appropriate view.
c. map one entity to its underlying table and one to the view.
Summary
Try (B) first, creating a separate view because it requires the least amount of change to both code and DB schema (you aren't fiddling with underlying tables, or affecting stored procedures). It also ensures your EF C# POCOs will function equivalently (one to a view and one to table may cause quirks). Miguel's answer below seems to be roughly the same suggestion so I would start here if it's possible.
Option (C) seems worst because your POCO entities may behave have unforseen quirks when mapped to different SQL pieces (tables vs. views) causing coding issues down the road.
Option (A), while it fits EF's intention best (entities mapped to tables), it means to get your joined view you must alter your C# services/repositories to work with the EF entities for Add, Update, Delete operations, but tell the Pull/Read-like methods to grab data from the joint views. This is probably your best choice, but involves more work than (B) and may also affect Schema in the long run. More complexity equals more risk.
Edit I'm not sure this is actually possible, and this is why:
The assumption is that a foreign key references a primary key. What you've got is two fields which are both acting as primary keys of vwContact, but depending on which object you ask it's a different field that's the primary key. You can only have one primary key at once, and although you can have a compound primary key you can't do primary key things with only half of it - you have to have a compound foreign key with which to reference it.
This is why Entity Framework doesn't have a way to specify the mapping column on the target side, because it has to use the primary key.
Now, you can layer some more objects on top of the EF entities to do some manual lookup and simulate the navigation properties, but I don't think you can actually get EF to do what you want because SQL itself won't do what you want - the rule is one primary key per table, and it's not negotiable.
From what you said about your database structure, it may be possible for you to write a migration script which can give the contact entities a consistent primary key and update everything else to refer to them with that single primary key rather than the two systems resulting from the legacy data, as you can of course do joins on any fields you like. I don't think you're going to get a seamlessly functional EF model without changing your database though.
Original Answer That Won't Work
So, vwContact contains a key KeyField which is referenced by many SomeObjects and another key LegacyKeyField which is referenced by many LegacyObjects.
I think this is how you have to approach this:
Give vwContact navigation properties for SomeObject and LegacyObject collections:
public virtual ICollection<SomeObject> SomeObjects { get; set; }
public virtual ICollection<LegacyObject> LegacyObjects { get; set; }
Give those navigation properties foreign keys to use:
modelBuilder.Entity<vwContact>()
.HasMany(c => c.SomeObjects)
.WithRequired(s => s.Contact)
.HasForeignKey(c => c.KeyField);
modelBuilder.Entity<vwContact>()
.HasMany(c => c.LegacyObjects)
.WithRequired(l => l.Contact)
.HasForeignKey(c => c.LegacyKeyField);
The trouble is I would guess you've already tried this and it didn't work, in which case I can't offer you much else as I've not done a huge amount of this kind of thing (our database is much closer to the kinds of thing EF expects so we've had to do relatively minimal mapping overrides, usually with many-to-many relationships).
As for your two calls to HasKey on vwContact, they can't both be the definitive key for the object, so it's either a compound key which features both of them, or pick one, or there's another field you haven't mentioned which is the real primary key. From here it's not really possible to say what the right option there is.
You should be able to do this with two different objects to represent the Contact view.
public class vwContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class vwLegacyContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class LegacyObject
{
public virtual vwLegacyContact Contact { get; set; }
public string ContactId { get; set; } //references vwLegacyContact.LegacyKeyField
}
ModelCreatingFunction(modelBuilder)
{
// can't set both of these, right?
modelBuilder.Entity<vwContact>().HasKey(x => x.KeyField);
modelBuilder.Entity<vwLegacyContact>().HasKey(x => x.LegacyKeyField);
// The rest of your configuration
}
I have tried everything that you can imagine, and found that most solutions won't work in this version of EF... maybe in future versions it supports referencing another entity by using an unique field, but this is not the case now. I also found two solutions that work, but they are more of a workaround than solutions.
I tried all of the following things, that didn't work:
Mapping two entities to the same table: this is not allowed in EF4.
Inheriting from a base that has no key definitions: all root classes must have keys, so that inherited classes share this common key... that is how inheritance works in EF4.
Inheriting from base class that defines all fields, including keys, and then use modelBuilder to tell wich base-properties are keys of the derived types: this doesn't work, because the methos HasKey, Property and others that take members as parameters, must reference members of the class itself... referencing properties of a base class is not allowed. This cannot be done: modelBuilder.HasKey<MyClass>(x => x.BaseKeyField)
The two things that I did that worked:
Without DB changes: Map to the table that is source of the view in question... that is, if vwContact is a view to Contacts table, then you can map a class to Contacts, and use it by setting the key to the KeyField, and another class mapping to the vwContacts view, with the key being LegacyKeyField. In the class Contacts, the LegacyKeyField must exist, and you will have to manage this manually, when using the Contacts class. Also, when using the class vwContacts you will have to manually manage the KeyField, unless it is an autoincrement field in the DB, in this case, you must remove the property from vwContacts class.
Changing DB: Create another view, just like the vwContacts, say vwContactsLegacy, and map it to a class in wich the key is the LegacyKeyField, and map vwContacts to the original view, using KeyField as the key. All limitations from the first case also applies: the vwContacts must have the LegacyKeyField, managed manually. And the vwContactsLegacy, must have the KetField if it is not autoincrement idenitity, otherwise it must not be defined.
There are some limitations:
As I said, these solutions are work-arounds... not real solutions, there are some serious implications, that may even make them undesirable:
EF does not know that you are mapping two classes to the same thing. So when you update one thing, the other one could be changed or not, it depends if the objects is cached or not. Also, you could have two objects at the same time, that represents the same thing on the backing storage, so say you load a vwContact and also a vwContactLegacy, changes both, and then try to save both... you will have to care about this yourself.
You will have to manage one of the keys manually. If you are using vwContacts class, the KeyFieldLegacy is there, and you must fill it. If you want to create a vwContacts, and associate is with a LegacyObject, then you need to create the reference manually, because LegacyObject takes a vwContactsLegacy, not a vwContacts... you will have to create the reference by setting the ContactId field.
I hope that this is more of a help than a disillusion, EF is a powerfull toy, but it is far from perfect... though I think it's going to get much better in the next versions.
I think this may be possible using extension methods, although not directly through EF as #Matthew Walton mentioned in his edit above.
However, with extension methods, you can specify what to do behind the scenes, and have a simple call to it.
public class LegacyObject
{
public virtual vwContact Contact { get; set; }
public string ContactId { get; set; } //references vwContact.LegacyKeyField
}
public class LegacyObjectExtensions
{
public static vwContact Contacts(this LegacyObject legacyObject)
{
var dbContext = new LegacyDbContext();
var contacts = from o in legacyObject
join c in dbContext.vwContact
on o.ContactId == c.LegacyKeyField
select c;
return contacts;
}
}
and
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class SomeObjectExtensions
{
public static vwContact Contacts(this SomeObject someObject)
{
var dbContext = new LegacyDbContext();
var contacts = from o in someObject
join c in dbContext.vwContact
on o.ContactId == c.KeyField
select c;
return contacts;
}
}
Then to use you can simply do like this:
var legacyContacts = legacyObject.Contacts();
var someContacts = someObject.Contacts();
Sometimes it makes more sense to map it from the other end of the relationship, in your case:
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).WithMany().HasForeignKey(u => u.LegacyKeyField);
however this will require that u.LegacyKeyField is marked as a primary key.
And then I'll give my two cents:
if the Legacy db is using LegacyKeyField, then perhaps the legacy db will be read only. In this case we can create two separate contexts Legacy and Non-legacy and map them accordingly. This can potentially become a bit messy as you'd have to remember which object comes from which context. But then again, nothing stops you from adding the same EF code first object into 2 different contexts
Another solution is to use views with ContactId added for all other legacy tables and map them into one context. This will tax performance for the sake of having cleaner context objects, but this can be counteracted on sql side: indexed views, materialized views, stored procs, etc. So than LEGACY_OBJECT becomes VW_LEGACY OBJECT with CONTACT.ContactId brought over, then:
modelBuilder.Entity<LegacyObject>().ToTable("VW_LEGACY_OBJECT");
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).WithMany().HasForeignKey(u => u.ContactId);
I personally would go with creating "mapper views" with CustomerId on legacy tables, as it's cleaner from c# layer perspective and you can make those views look like real tables. It is also difficult to suggest a solution without knowing what exactly is the scenario that you have a problem with: querying, loading, saving, etc.
In an application I'm working on, I have what are essentially a bunch of lookup tables in a database which all contain two things: The ID (int) and a Value (string).
There's only a handful of them, but I want to map all of them to a single Context which depends on the table name. Something like:
class LookupContext : DbContext
{
public DbSet<Lookup> Lookups { get; set; }
public LookupContext(String table)
{
// Pseudo code:
// Bind Lookups based on what table is
Lookups = MyDatabase.BindTo(table);
}
}
So if I create a new LookupContext("foo"), it binds against the foo table. If I do new LookupContext("bar") it uses the bar table, and so forth.
Is there any way to do this? Or do I have to create a separate context + model for every table I have?
This is more or less my first time doing this, so I'm not really sure if what I'm doing is right.
The answer we should be able to give you is to use enums, but that's not available quite yet - it's in the next version of EF. See here for details: http://blogs.msdn.com/b/adonet/archive/2011/06/30/walkthrough-enums-june-ctp.aspx
With earlier versions of EF, you can simply create a class per lookup value (assuming state as an example) and have code that looks something like the following:
public class State
{
public int StateId {get;set;}
public string StateName {get;set;}
}
public class LookupContext : DbContext
{
public DbSet<State> States {get;set;}
// ... more lookups as DbSets
}
This will allow you to use one context but will still require one class per table. You can also use the fluent API if you want your table/column names to differ from your class/property names respectively. Hope that helps!
I actually realized I was completely over complicating things beyond reason. There was no reason for storing multiple tables with two columns.
I'm better off storing my data as:
public class LookupValue
{
public string LookupValueId { get; set; }
public string Value { get; set; }
public string LookupType { get; set; }
}
Where the third field was simply the name of the table that I was previously storing in the database.
I'm still interested in the idea of mapping a single Context class to multiple tables, but I believe what I described above is the least convoluted way of accomplishing what I need.