Matthieu MEZIL

How to simulate a 1 to 0..1 relationship when you must have a 1 to 1

Matthieu MEZIL — Fri, 14 Nov 2008 10:31:00 GMT

If you take the Noam edm, you have a 1 to 1 relationship that you must have because the relationship is on the PK of the two entities which are the same.

Now imagine that in his sample, FrontImage, TopImage and SideImage are nullable.

We can imagine creating a new Product without a new ProductImages.

If you do this, you will have an exception on the SaveChanges because you have a 1 to 1 relationship so you must have a ProductImages associated.

My idea is to automaticaly create the ProductImages (if it does'nt exist) on the SaveChanges.

What I suggest is to extend the ObjectContext, to use the partial method OnContextCreated to add handler to the SavingChanges event:

partial class Test4125924Entities
{
    partial void OnContextCreated()
    {
        SavingChanges += (sender, e) =>
        {
            foreach (var product in ObjectStateManager.GetObjectStateEntries(EntityState.Added).Select(ose => ose.Entity).OfType<Product>())
                if (product.ProductImages == null)
                    product.ProductImages = new ProductImages();
        };
    }
}

How to split a data table?

Matthieu MEZIL — Thu, 13 Nov 2008 21:06:00 GMT

In this msdn thread, zeeshan hirani wants to split a data table in two entities to do a sort of delay loading?

So how to do this?

I do this with Northwind:

<!-- EF Runtime content -->
<edmx:Runtime>
    <!-- SSDL content -->
    <edmx:StorageModels>
        <Schema Namespace="NorthwindModel.Store" Alias="Self" Provider="System.Data.SqlClient" ProviderManifestToken="2008" xmlns:store="http://schemas.microsoft.com/ado/2007/12/edm/EntityStoreSchemaGenerator" xmlns="http://schemas.microsoft.com/ado/2006/04/edm/ssdl">
            <EntityContainer Name="NorthwindModelStoreContainer">
                <EntitySet Name="Categories" EntityType="NorthwindModel.Store.Categories" store:Type="Tables" Schema="dbo" />
            </EntityContainer>
            <EntityType Name="Categories">
                <Key>
                    <PropertyRef Name="CategoryID" />
                </Key>
                <Property Name="CategoryID" Type="int" Nullable="false" StoreGeneratedPattern="Identity" />
                <Property Name="CategoryName" Type="nvarchar" Nullable="false" MaxLength="15" />
                <Property Name="Description" Type="nvarchar" MaxLength="200" />
                <Property Name="Picture" Type="image" />
            </EntityType>
        </Schema>
    </edmx:StorageModels>
    <!-- CSDL content -->
    <edmx:ConceptualModels>
        <Schema Namespace="NorthwindModel" Alias="Self" xmlns="http://schemas.microsoft.com/ado/2006/04/edm">
            <EntityContainer Name="NorthwindEntities">
                <EntitySet Name="CategorySet" EntityType="NorthwindModel.Category" />
                <EntitySet Name="CategoryPictureSet" EntityType="NorthwindModel.CategoryPicture" />
                <AssociationSet Name="CategoryCategoryPicture" Association="NorthwindModel.CategoryCategoryPicture">
                    <End Role="Category" EntitySet="CategorySet" />
                    <End Role="CategoryPicture" EntitySet="CategoryPictureSet" />
                </AssociationSet>
            </EntityContainer>
            <EntityType Name="Category">
                <Key>
                    <PropertyRef Name="CategoryID" />
                </Key>
                <Property Name="CategoryID" Type="Int32" Nullable="false" />
                <Property Name="CategoryName" Type="String" Nullable="false" MaxLength="15" Unicode="true" FixedLength="false" />
                <Property Name="Description" Type="String" MaxLength="200" Unicode="true" FixedLength="false" />
                <NavigationProperty Name="Picture" Relationship="NorthwindModel.CategoryCategoryPicture" FromRole="Category" ToRole="CategoryPicture" a:SetterAccess="Private" xmlns:a="http://schemas.microsoft.com/ado/2006/04/codegeneration" />
            </EntityType>
            <EntityType Name="CategoryPicture">
                <Key>
                    <PropertyRef Name="CategoryID" />
                </Key>
                <Property Name="CategoryID" Type="Int32" Nullable="false" a:GetterAccess="Private" xmlns:a="http://schemas.microsoft.com/ado/2006/04/codegeneration" a:SetterAccess="Private" />
                <NavigationProperty Name="Category" Relationship="NorthwindModel.CategoryCategoryPicture" FromRole="CategoryPicture" ToRole="Category" />
                <Property Name="Picture" Type="Binary" Nullable="true" />
            </EntityType>
            <Association Name="CategoryCategoryPicture">
                <End Type="NorthwindModel.Category" Role="Category" Multiplicity="1" />
                <End Type="NorthwindModel.CategoryPicture" Role="CategoryPicture" Multiplicity="1" />
                <ReferentialConstraint>
                    <Principal Role="Category">
                        <PropertyRef Name="CategoryID" />
                    </Principal>
                    <Dependent Role="CategoryPicture">
                        <PropertyRef Name="CategoryID" />
                    </Dependent>
                </ReferentialConstraint>
            </Association>
        </Schema>
    </edmx:ConceptualModels>
    <!-- C-S mapping content -->
    <edmx:Mappings>
        <Mapping Space="C-S" xmlns="urn:schemas-microsoft-com:windows:storage:mapping:CS">
            <EntityContainerMapping StorageEntityContainer="NorthwindModelStoreContainer" CdmEntityContainer="NorthwindEntities">
                <EntitySetMapping Name="CategorySet">
                    <EntityTypeMapping TypeName="IsTypeOf(NorthwindModel.Category)">
                        <MappingFragment StoreEntitySet="Categories">
                            <ScalarProperty Name="CategoryID" ColumnName="CategoryID" />
                            <ScalarProperty Name="CategoryName" ColumnName="CategoryName" />
                            <ScalarProperty Name="Description" ColumnName="Description" />
                        </MappingFragment>
                    </EntityTypeMapping>
                </EntitySetMapping>
                <AssociationSetMapping Name="CategoryCategoryPicture" TypeName="NorthwindModel.CategoryCategoryPicture" StoreEntitySet="Categories">
                    <EndProperty Name="CategoryPicture">
                        <ScalarProperty Name="CategoryID" ColumnName="CategoryID" />
                    </EndProperty>
                    <EndProperty Name="Category">
                        <ScalarProperty Name="CategoryID" ColumnName="CategoryID" />
                    </EndProperty>
                </AssociationSetMapping>
                <EntitySetMapping Name="CategoryPictureSet">
                    <EntityTypeMapping TypeName="IsTypeOf(NorthwindModel.CategoryPicture)">
                        <MappingFragment StoreEntitySet="Categories">
                            <ScalarProperty Name="Picture" ColumnName="Picture" />
                            <ScalarProperty Name="CategoryID" ColumnName="CategoryID" />
                        </MappingFragment>
                    </EntityTypeMapping>
                </EntitySetMapping>
            </EntityContainerMapping>
        </Mapping>
    </edmx:Mappings>
</edmx:Runtime>
 

Then, I extend (partial) the CategoryPicture class to do a constructor with a Category parameter. Indeed, we can’t create a CategoryPicture without a Category because CategoryName doesn’t allow null.

partial class CategoryPicture
{
    public CategoryPicture(Category category)
    {
        Category = category;
        CategoryID = category.CategoryID;
    }
}

How to dress up to a wedding?

Matthieu MEZIL — Thu, 06 Nov 2008 10:32:00 GMT

When you are invited to a wedding, you have to be fashionable. But if the groom works in computing, an important grand shirt is a little too conservative and the best is this:

Get added entities in the query (next)

Matthieu MEZIL — Mon, 03 Nov 2008 08:08:00 GMT

I want to complete my yesterday post.

If you just want to get the entities Count, it is useless to load all the entities in memory.

So in this case, it's better to do something like this:

context.MyEntitySet.Count() + context.ObjectStateManager.GetObjectStateEntries(EntityState.Added).Select(e => e.Entity).OfType<MyEntityType>().Count() - context.ObjectStateManager.GetObjectStateEntries(EntityState.Deleted).Select(e => e.Entity).OfType<MyEntityType>().Count()

C# compiler and implicit keyword: can be better

Matthieu MEZIL — Mon, 03 Nov 2008 07:51:00 GMT

Imagine the following code:

class A
{
    public static implicit operator B(A a)
    {
        return new B(a);
    }
}
class B
{
    public B(A a)
    {
    }
}

This code compile without error and it's normal.

Now imagine the following one:

class A
{
    public static implicit operator B(A a)
    {
        return new A();
    }
}

The compiler generates no error with this code but it should!

Less annoying, the compiler generates an error with this code (Cannot implicitly convert type 'A' to 'C') even though it would be able to use B to convert A to C:

class Program
{
    static void Main(string[] args)
    {
        C c = new A();
    }
}
 
class A
{
    public static implicit operator B(A a)
    {
        return new B(a);
    }
}
class B
{
    public B(A a)
    {
    }
 
    public static implicit operator C(B b)
    {
        return new C(b);
    }
}
 
class C
{
    public C(B b)
    {
    }
}

Get added entities in the query

Matthieu MEZIL — Sun, 02 Nov 2008 14:36:00 GMT

I had the following question:

Why, when we add an entity in an ObjectContext, don't we get it from a query until we save the ObjectContext?

For example,

context.MyEntities.Count returns n.

Now, if we add a new entity in the context, context.MyEntities.Count still returns n, not n + 1.

In fact, when we execute a query, the query is translated into a sql query and so, the result is the result of the database and until you call the SaveChanges method on the context, the Added or Deleted entities are ignored.

So how to consider the EntityState?

We can use the ObjectStateManager.

You have to load all the entities you need from the DB to have them in the ObjectContext's ObjectStateManager.

Then, you can get the entities with:

context.ObjectStateManager.GetObjectStateEntries(EntityState.Added | EntityState.Modified | EntityState.Unchanged).Select(e => e.Entity).OfType<MyEntityType>()

ADO.NET Data Services Hooking POC V4

Matthieu MEZIL — Mon, 27 Oct 2008 23:38:00 GMT

I don't like rights management with ADO.NET Data Services.

Indeed, you can use a method with ChangeInterceptor attribute to manage CUD operations and a QueryInterceptor attribute to manage Read operations. But if it's a right logic, I think it should be in the entity class (in the BLL) and not in the service.

With my POC, you just need the entity type implements one or two interfaces:

public interface IEntityNeedRightToRead
{
    bool CanRead(string login);
}
public interface IEntityNeedRightsToWrite
{
    bool CanChange(string login, string propertyName);
    bool CanChangeLinks(string login, string propertyName);
    bool CanAdd(string login);
    bool CanDelete(string login);
}

Moreover, in my sample, I have the Login property in my entity types so it's easy to return the default value of a property if you don't want to expose it for the connected people.

The next step is, in this case, to exclude the property if it is in ETags.