# IT:AD:Code First:DbContext #



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<panel title="Summary">

An EF `DbContext` is an "occasionally-connected",  "in-memory" representation of the remote database.

In other words, in its simplest form, a `DbContext` is rather like a `DataSet` of the whole database, keeping track of changes made to its in-mem tables, and only committing those changes back to the database when the `Commit()` method is called.


</panel>


## Process ##


### Recipe ###

The recipe for creating an application specific DbContext is as follows.

* Create an in-app class that derives from `DbContext` (it's an `abstract` class).
* Create an appropriate constructor (see: [[#About the ConnectionString/]])
* Choose an initialization strategy (see [[#About Initialization/]]) and optionally deal with Seeding concerns at the same time.
* Optionally provide one or more `DbSet<TEntity>` representations of database tables.


<sxh csharp>

    public class TestContext : DbContext {
    
       //See: http://bit.ly/t7r4pS
          public TestContext():base("OverrideWithCustomConnStrNameThatDiffersFromExpectedTestContex") {}

          public DbSet<TestEntity> TestEntities { get; set; }

          protected override void OnModelCreating(DbModelBuilder modelBuilder) {

           //Keep tables as TestEntity -- not TestEntities
           modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();

            //modelBuilder.Entity<TEstEntity>()
            //  .HasRequired(a => a.Source)
            //  .WithMany()
            //  .HasForeignKey(a => a.SourceId);
  
        }
    }
    
</sxh>


### About the ConnectionString ###

As stated above, the most common common convention is to provide an argument-less constructor (note that an argumentless constructor is required for Code migrations from a [[IT/AD/Continuous Integration/]] point of view).

The reasoning behind this is that by default the `DbContext` finds its connection string by adhering to the [[IT/AD/Design/Patterns#Convention over Configuration/]] pattern: 

<sxh xml>
  <add name="ConnStrName"
     connectionString="Data Source=.;Initial Catalog=SomeDb;Integrated Security=True"
     providerName="System.Data.SqlClient"
     />
</sxh>

Note that it actually looks for the best solution in several places: http://bit.ly/t7r4pS


<WRAP tip>
For those who have a need to go beyond *Convention* (really?), one can override the *ConnectionString* with a custom name (see below) or passing a specific ConnectionString (it will tell the difference by the fact that *ConnectionString*'s have a '=' in them somewhere.  

But before breaking Conventions, consider that they they are there for a reason. For one, they're cheaper to maintain among a team.
</WRAP>


### About Initialization ###

The first call to the DbContext implementation during the lifespan of the application will invoke the [[IT/AD/Code First/DbContext/Initialization|IDatabaseInitializer]] associated to the database.

There are various [[IT/AD/Code First/DbContext/Initialization|IDatabaseInitializer]], but I think they all check to see if the database schema has changed before proceeding. They do this by querying the schema from the database, and comparing this against an in-mem map of what your code says the database is. You provide the in-mem description of the map within  the `OnModelCreating` event handler. Depending on the flavour of the [[IT/AD/Code First/DbContext/Initialization|IDatabaseInitializer]] invoked, if the two don't match, it may decide to upgrade the db, using the model you provided within the `OnModelCreating` event handler.

The Model is built up using [[IT/AD/Code First/FluentAPI/]] syntax.

Once the model check within the [[IT/AD/Code First/DbContext/Initialization|IDatabaseInitializer]] stage is out of the way (it's only relevant for the first call of an app), you refer to Database tables by exposing DbSet<T> properties of those entities (exposing entities that are not described within the `OnModelCreating` event handler will obviously create an error).




### Notes ###

Note that unlike *ModelFirst* or *DatabaseFirst* (which expect [EntityConnection](http://bit.ly/uPULxy)), *CodeFirst* expects just a *ConnectionString*.