Allow creation of a DbMigrator with an explicit DbContext instance

This prevents migrations having to attempt to create connections from connection strings.

Fixes #522

src/EntityFramework/Migrations/DbMigrator.cs

@@ -94,10 +94,17 @@ public DbMigrator(DbMigrationsConfiguration configuration)
             Check.NotNull(configuration.ContextType, "configuration.ContextType");
         }
 
+        /// <summary>
+        /// Initializes a new instance of the DbMigrator class using the supplied context.
+        /// Use this constructor when applying migrations from code to avoid having migrations attempt
+        /// to create a context/connection for you.
+        /// </summary>
+        /// <param name="configuration"> Configuration to be used for the migration process. </param>
+        /// <param name="context"> The <see cref="DbContext"/> to use. </param>
         [SuppressMessage("Microsoft.Maintainability", "CA1506:AvoidExcessiveClassCoupling")]
         [SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
-        internal DbMigrator(DbMigrationsConfiguration configuration, DbContext usersContext)
-            : this(configuration, usersContext, DatabaseExistenceState.Unknown, calledByCreateDatabase: false)
+        public DbMigrator(DbMigrationsConfiguration configuration, DbContext context)
+            : this(configuration, context, DatabaseExistenceState.Unknown, calledByCreateDatabase: false)
         {
         }
 

test/EntityFramework/FunctionalTests/Migrations/NoTestInfraScenarios.cs

@@ -2,11 +2,14 @@
 
 namespace System.Data.Entity.Migrations
 {
+    using System.Data.Common;
     using System.Data.Entity.Infrastructure;
     using System.Data.Entity.Migrations.Design;
     using System.Data.SqlClient;
     using System.Linq;
+    using System.Reflection;
     using System.Threading.Tasks;
+    using SimpleModel;
     using Xunit;
 
     public class NoTestInfraScenarios : TestBase
@@ -33,6 +36,41 @@ var migrator
             Assert.False(string.IsNullOrWhiteSpace(migration.Directory));
         }
 
+        [Fact]
+        public void Can_generate_migration_from_user_code_without_cloning_connection()
+        {
+            var connectionString = SimpleConnectionString("Db471a");
+
+            using (var context = new EmptyContext(connectionString))
+            {
+                context.Database.Delete();
+            }
+
+            using (var context = new Context471(connectionString))
+            {
+                var migrationConfiguration = new Configuration471(connectionString);
+                var migrator = new DbMigrator(migrationConfiguration, context);
+
+                var connectionField = migrator.GetType().GetField(
+                    "_connection", 
+                    BindingFlags.Instance | BindingFlags.NonPublic);
+
+                Assert.Same(context.Database.Connection, connectionField.GetValue(migrator));
+
+                migrator.Update();
+
+                Assert.Same(context.Database.Connection, connectionField.GetValue(migrator));
+
+                migrator = new DbMigrator(migrationConfiguration, context);
+
+                Assert.Same(context.Database.Connection, connectionField.GetValue(migrator));
+
+                migrator.Update();
+
+                Assert.Same(context.Database.Connection, connectionField.GetValue(migrator));
+            }
+        }
+
         [Fact] // CodePlex #2579
         public void Repro()
         {
@@ -97,6 +135,49 @@ public Context2579(string nameOrConnectionString)
             public DbSet<Entity2579> Entities { get; set; }
         }
 
+        public class Context471 : DbContext
+        {
+            static Context471()
+            {
+                Database.SetInitializer<Context471>(null);
+            }
+
+            public Context471()
+            {
+            }
+
+            public Context471(string nameOrConnectionString)
+                : base(CreateConnection(nameOrConnectionString), true)
+            {
+            }
+
+            private static DbConnection CreateConnection(string connectionString)
+            {
+                return new SqlConnection(connectionString);
+            }
+
+            public DbSet<Entity471> Entities { get; set; }
+        }
+
+        public class Entity471
+        {
+            public int Id { get; set; }
+        }
+
+        internal sealed class Configuration471 : DbMigrationsConfiguration<Context471>
+        {
+            public Configuration471()
+            {
+                AutomaticMigrationsEnabled = true;
+            }
+
+            public Configuration471(string connectionString)
+            {
+                AutomaticMigrationsEnabled = true;
+                TargetDatabase = new DbConnectionInfo(connectionString, "System.Data.SqlClient");
+            }
+        }
+
         public class Context2579I : DbContext
         {
             static Context2579I()