Assume this model and a key-value store that supports to take whole objects and not only scalar types:
class Country
{
public string Name { get; set; }
}
class Company
{
public string Key { get; set; }
public string Name { get; set; }
public Country Country { get; set; }
}
var bmw = new Company
{
Key = "bmw",
Name = "Bayerische Motoren Werke",
Country = new Country { Name = "Germany" }
};
var vw = new Company
{
Key = "vw",
Name = "Volkswagen",
Country = new Country { Name = "Germany" }
};
myStore.Put(bmw.Key, bmw);
myStore.Put(vw.Key, vw);
Questions:
How would I deal with the relation between Company and Country without having redundant data?
Company.Key: Is it a good idea to store the key inside the value? Technically it is redundant and therefore 'bad'. But after passing a 'Company' around in a multi layer application, I might need to remember the related key.
Consider the following example of key/value pairs -- using not a numeric identifier but a meaningful, standards-body-assigned primary key:
Country.de.Name=Germany
Company.1.Name=BMW
Company.1.Country=de
Company.2.Name=VW
Company.2.Country=de
This is meaningfully normalized as respecting countries: The schema design is such that country name is only stored once, and can only be stored once (preventing any potential for conflicting data).
"Integrity checks" and "foreign keys" are unrelated concepts to normalization; that said, I will note that some tuple stores (of which key/value stores are effectively a degenerate subset) do support constraint enforcement; in Datomic, for example, this is implemented via transaction functions.
Redis, likewise, can have arbitrary Lua code run server-side, a capability which applications could use for constraint enforcement. In short: It is possible to have meaningful normalization for content in key/value stores.
Related
My scenario:
I have a Product that has various properties such a price, size, etc. that are declared in the Product Entity.
Additionally, a Product can have a collection of StockRequirements, i.e. when that Product is used the constituent StockItems can be depleted by the StockRequirement quantity accordingly.
Under one use case I just want the Product so that I can play with the core properties. For another use case I want the Product with its StockRequirements.
This means that when retrieving a Product I may be using it in different contexts. My chosen approach has been to use EF table splitting.
I have one repository for Products and one repository for ProductStockRequirements. They are referring to the same unique Product.
The Product repository will provide a Product Entity with the core details only.
The ProductStockRequirements repository will provide ProductStockRequirements entity which does not have the core details, but does have the list of StockRequirements.
This seemed a reasonable approach so that I am not retrieving 'owned' StockRequirements when I only want to change the price of the product. Similarly, if I'm only interested in playing with the StockRequirements then I don't retrieve the other core details.
Entities
class Product
{
public int Id { get; set; }
public string CoreProperty { get; set; }
}
class ProductStockRequirements
{
public int Id { get; set; }
public List<StockRequirement> StockRequirements { get; set; }
}
Product Mapping
b.ToTable("Products");
b.HasKey(p => p.Id);
b.Property(p => p.CoreProperty).IsRequired();
ProductStockRequirementsMapping
b.ToTable("Products");
b.HasKey(p => p.Id);
b.OwnsMany<StockRequirement>(p => StockRequirements, b =>
{
b.ToTable("StockRequirements");
b.WithOwner().HasForeignKey("ProductId");
}
b.HasOne<Product>()
.WithOne()
.HasForeignKey<ProductStockRequirements>("Id");
When running a migration, I get the warning:
The entity type 'ProductStockRequirements' is an optional dependent
using table sharing without any required non shared property that
could be used to identify whether the entity exists. If all nullable
properties contain a null value in database then an object instance
won't be created in the query. Add a required property to create
instances with null values for other properties or mark the incoming
navigation as required to always create an instance.
Focusing on the advice:
mark the incoming navigation as required to always create an instance
I have tried:
b.HasOne<Product>()
.WithOne()
.HasForeignKey<ProductStockRequirements>("Id")
.IsRequired();
and
b.HasOne<Product>()
.WithOne()
.IsRequired()
.HasForeignKey<ProductStockRequirements>("Id");
to no avail.
The warning does not appear to result in any bad behaviour. All my tests are passing. But, it seems that I should be able to create a map that removed this warning, but cannot find the way.
This should really just be
class Product
{
public int Id { get; set; }
public string CoreProperty { get; set; }
public List<StockRequirement> StockRequirements { get; set; } = new List<StockRequirement>();
}
As the StockRequiremens are not part of the Product entity, and related data isn't loaded unless you request it.
And the Entity model is simply not the correct layer to define your aggregates. An Aggregate is defined by selecting a single Entity from your entity model along with 0-few related entities. Typically you include the closely-related and weak entities together in an aggregate.
If your entity model is a graph of 23 related entities, you might organize it into 10 separate and partially-overlapping aggregates or sub-graphs.
I have one entity were I describe Document Type :
public class DocumentType
{
public int Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public bool IsLogical { get; set; }
}
You can see the boolean IsLogical. If it set to true, It means it's a document which point to a list of DocumentType where IsLogical is set to false (physical document). The objective is to declare a document like "Identity document" and point it to "Id Card" and "Passport" and "Driving licence" for example.
I don't find to good way to do it. I found three different ways but I'm not confortable with :
Declare another entity
I can declare another entity like "LogicalToPhysical" with only foreign keys but I don't know if EF Core can manage something like this : LogicalDocumentTypeId is an Id from DocumentType (FK) where IsLogical is set to true and PhysicalDocumentTypeId is an Id from DocumentType (another FK) where IsLogical is set to false (one to many relationship).
Using Inherance
According to this, I can use inherance but I don't know how set the other entity in order to manage my one to many relationship. Another problem is to manage two kind of objects.
Using value Object / Owned Entity types
If I understood properly this article, I can using Value object in order to map my list, but I'm not sure how do this.
Maybe my conception is very bad, but if I can avoid to manage differents objects (and query from different sources), it would be better.
I'm using Entity Framework Core 5 in a net core API project with SQL Server 2019.
EDIT : in my reasoning, I imagine something like that :
Example of DocumentType
{1, 'Toto', 'blabla', false }
{2, 'Titi', 'blabla', false }
{3, 'Tutu', 'blabla', true }
{4, 'Tata', 'blabla', false }
Id 3 --> IsLogical : true
Example of another table (LogicalDocument for example)
LogicalDocument {DocumentType.Id FK, DocumentType.Id FK)
{ 3, 1 } // Tutu is pointing to Toto
{ 3, 2 } // Tutu is pointing to Titi
If I have two tables, which have an Id, whish is an autogenerated int (seed), anyway I have a many to many relationship between these two tables which requires another table.
Now, I do a "dry run" to generate the items for the first two table before saving them, this works perfect. The problem is when I try to generate the items for the (many-many relationship) in the third table. Before saving the items all Ids in the first two tables will be set to 0, when adding items to the relation table I have no problems, the problems comes when saving the tables because the relationship table will have the Ids of 0.
Is there a way to overcome this problem? like assigning a temp value which will be automatically changed to the real Id in the relationship table before saving it ?
For the same reason, I've chosen not to use default Seed methods (AddOrUpdate) provided by EF, but I'm rather writing my own seed methods.
Now if I want to set up relationships, I'm not explicitly using ID's, but rather use navigational properties.
Imagine the scenario:
public class User
{
public int Id {get;set;}
public string Name {get;set;}
public virtual IList<Roles> Roles { get;set;}
}
public class Role
}
public int Id {get;set;}
public string Name {get;set;}
public virtual IList<User> Users {get;set;}
}
Doing this will seed both values for user and roles and relationships once you hit the save changes:
Role admin = new Role
{
Name = "Administrator"
};
Role basic = new Role
{
Name = "Basic"
};
User user = new User
{
Name = "John",
Roles = new List<Role>()
{
basic,
admin
}
}
db.Users.Add(user);
db.SaveChanges();
I have an existing database that I cannot change, but want to access using EF. For 90% of the database, I have been able to get Code First EF to work, and I am very impressed.
I've run into a case that I am wondering how to model or access the data through a navigation property.
In one case, the tables are like this (this example is totally made up, but represents the problem):
CREATE TABLE Dog (
id INTEGER PRIMARY KEY,
name VARCHAR(50) NULL,
breed_id INTEGER NOT NULL
);
CREATE TABLE Breed (
id INTEGER NOT NULL,
organization_id INTEGER NOT NULL,
description VARCHAR(100) NOT NULL,
Primary key (id, organization)
);
CREATE TABLE Organization (
id INTEGER PRIMARY KEY,
description VARCHAR(100) NOT NULL
);
In Table breed, the organization represents an organization that has defined a breed. A dog can have several breeds, but the program only displays one, the results being 'filtered' by the organization id - which is a value that is configured when the program is set up.
An example of the data that might be present is this:
id organizationID Description
1 1 Basset Hound
2 1 Great Dane
2 2 Grande Dane
Where organization 2 has chosen to call the breed something different than organization 1. The unique primary key is a combination of id and organizationID. A dog has a breed, but does not have a property to define one or more associated organizations. It takes additional information from another table, or a configured value (perhaps an enumerated value) to find the breed of a dog.
In my case, to find a particular dog breed, you have to have a dog id and another piece of information (organization_id) which is related to program configuration.
The dog, breed and organization classes look like this:
public class Dog {
public int id { get; set; };
public string name { get; set; };
public int breedID { get; set; };
public virtual Breed { get; set; }
}
public class Breed {
public int id { get; set; };
public int organizationID { get; set; };
public string description { get; set; };
public virtual Organization { get; set; }
}
public class Organization {
public int id { get; set; };
public string description { get; set; };
}
As seen in the code, I'd like to use a "Navigation" Property on Dog that returns a breed, but don't think I can configure this in code first.
I've tried a few different things (in fluent API, and leaving organization out - since that's easy) and will also document things I don't think work:
1)
modelBuilder.Entity<Dog>().HasKey(t => t.id);
modelBuilder.Entity<Breed>().HasKey(t => t.id);
modelBuilder.Entity<Dog>().HasRequired(d => d.Breed).WithMany().HasForeignKey(d => d.breedID);
Of course, the problem with this is that more than one breed will be returned and entity framework will throw an exception because the breedID itself is insufficient to yield a single value - which the model is calling for.
2)
Change class dog:
Remove:
public virtual Breed { get; set; }
Add:
public virtual ICollection<Breed> Breeds { get; set; }
public Breed Breed {
get {
// Assume 1 is configured organization value
return Breeds.Single(t => t.OrganizationId == 1);
};
set {
Breeds.Add(value);
}
}
Change model:
I don't know how to do this for the given classes. Since it's a Collection, it must look something like
modelBuilder.Entity<Dog>().HasMany(d => d.Breeds)...
but I don't see how to specify that breedID is a foriegn key into the breed table.
If I could get the model to work, the rest will work, but it does seem wierd and inefficient.
3)
Change model to account for composite key (using first set of classes):
modelBuilder.Entity<Breed>().HasKey(b => new { b.id, b.organizationId });
modelBuilder.Entity<Dog>().HasRequired(t => t.Breed).WithMany().HasForeignKey(t => new { t.breedID, configuredValue });
I don't know how to "inject" configuredValue as in the last line, so this doesn't work either.
If none of the above methods work, or if I can't find another way to configure code first properly, then I'd like to specify that when the Breed navigation property getter is called, it should use a query that can get the appropriate breed and return it appropriately.
However, I don't want to dirty my POCO with the Context calls to return the result of the query. In other words, I'd like to have a property on Dog that does NOT look something like this:
public Breed Breed {
get {
return context.Breed.Where(b => b.id == this.id && b.organizationID == 1).Single();
}
}
Ideally, it would work like the Navigation collections do, where EF does it's magic and returns the appropriate results.
Intuitively, it seems like I should be able to either configure this using POCO-like code or use/extend a proxy to extend the configuration to use the particular query I want when the accessor is called. Or - it seems like I ought to be able to populate the property on any read and dirty the POCO on write. I'm just not familiar enough with EF to know how to do this.
Is this possible?
As an addition to the first post, because I desire to keep my POCO classes clean, I think I will probably implement the Repository pattern to encapsulate the complex queries like the ones I've described, as well as support other operations also.
Looking at the EDMX that I've generated from my Code First model, it's not apparent how to implement the model from a database first perspective, either.
Any thoughts are greatly appreciated.
In short, you can't do that. You have illogical database structure (foreign key referencing non-unique column), you'll have to map it somewhere. You can't do that in EF configuration, because it's dynamic, so you'll have to do that inside you entity classes. And I see no ways to do that without direct call to context inside your entity class (or helper class).
Given this:
create table Location(
LocationId int identity(1,1) not null primary key,
Address nvarchar(max) not null,
City nvarchar(max) null,
State nvarchar(max) not null,
ZipCode nvarchar(max) not null
);
create table Park(
ParkId int not null primary key references Location(LocationId),
Name nvarchar(max) not null
);
I tried this mapping:
modelBuilder.Entity<Location>();
modelBuilder.Entity<Park>().ToTable("Park");
modelBuilder.Entity<Park>().Property(x => x.LocationId).HasColumnName("ParkId");
Unfortunately that didn't work.
using (var db = new Ef())
{
var park = new Park { Name = "11th Street Park", Address = "801 11th Street", City = "Aledo", State = "TX", ZipCode = "76106" };
db.Set<Location>().Add(park);
db.SaveChanges();
}
It has this error:
The property 'LocationId' is not a declared property on type 'Park'.
Verify that the property has not been explicitly excluded from the
model by using the Ignore method or NotMappedAttribute data
annotation. Make sure that it is a valid primitive property.
How should I map Park entity so its LocationId property fall to ParkId column?
I have this mapping by the way:
public class Location
{
public virtual int LocationId { get; set; }
public virtual string Address { get; set; }
public virtual string City { get; set; }
public virtual string State { get; set; }
public virtual string ZipCode { get; set; }
}
public class Park : Location
{
public virtual string Name { get; set; }
}
If it could help, this is possible in EF 4.0 (via designer), just followed the steps in Chapter 2-11 of Entity Framework 4.0 Recipes, Problem Solution Approach. Now I'm trying it on code first via EF 4.1
[EDIT]
If I change the ParkId to LocationId, things are ok. However, with designer approach, it is possible to map the LocationId to ParkId of table Park; I want to achieve the same thing with code first
create table Park(
LocationId int not null primary key references Location(LocationId),
Name nvarchar(max) not null
);
As I know (and I tried it multiple times) code first doesn't support this => your derived type should use same column names for primary key.
This problem can be described very simply: Current fluent mapping implementation doesn't allow overriding mapping rules from parent entity => parent entity defines names of primary key columns in all derived entities.
IMO the most probable reason is that it was really designed as code first where you don't have existing database and you do not have to bother with database naming - it was up to EF to define names as it needed. Once DbContext API was released people started to use it with existing database massively. But here comes a problem: Initial use cases didn't count with this so some scenarios which are pretty easily done in EDMX are not possible. This is one of them.
Here is a workaround for this issue:
Create a view for the derived table and map your entity class that view. Rename the key column in your view so that it matches the key column in the base table.
eg:
base table User (UserID, FirstName, LastName)
derived table Manager (ManagerID, DepartmentID)
Entity Framework fails to update Manager as the key column is different!
solution:
create view UserManager
as
select
ManagerID as UserID,
DepartmentID
from Manager
Then map the Manager class to the UserManager view, instead of to the Manager table.