Entity Framework 4 "Code-First": Custom Database Schema Mapping
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by Scott Guthrie
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Scenario 3: Splitting a Table Across Multiple Types

Relational tables within a database are often structured differently than how you want to design your object-oriented model classes.  What might be persisted as one large table within a database is sometimes best expressed across multiple related classes from a pure object-oriented perspective – and often you want the ability to split or shred tables across multiple objects related to a single entity.

For example, instead of a single “colAddr” column for our address, let’s assume our “tblDinners” database table uses multiple columns to represent the “address” of our event:

image

Rather than surface these address columns as 4 separate properties on our “Dinner” model class, we might instead want to encapsulate them within an “Address” class and have our “Dinner” class exposes it as a property like so:

image

Notice above how we’ve simply defined an “Address” class that has 4 public properties, and the “Dinner” class references it simply by exposing a public “Address” property.  Our model classes are pure POCO with no persistence knowledge.

We can update our “OnModelCreating” method to support a mapping of this hierarchical class structure to a single table in the database using a rule like so:

image

Notice how we are using the same mapping approach we used in the previous example – where we map table column names to strongly-typed properties on our model object.  We are simply extending this approach to support complex sub-properties as well.  The only new concept above is that we are also calling modelBuilder.ComplexType<Address>() to register our Address as a type that we can use within mapping expressions.

And that is all we have to write to enable table shredding across multiple objects.

Download an Updated NerdDinner Sample with Custom Database Persistence Rules

You can download an updated version of the NerdDinner sample here.  It requires VS 2010 (or the free Visual Web Developer 2010 Express).

You must download and install SQL CE 4 on your machine for the above sample to work.  You can download the EF Code-First library here.  Neither of these downloads will impact your machine.


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