DontDropDBJustCreateTables Entity Framework code first helper for AppHarbor by Joachim Lykke Andersen

DontDropDBJustCreateTables.cs

// Original code by Joachim Lykke Andersen; http://devtalk.dk/2011/02/16/Workaround+EF+Code+First+On+AppHabour.aspx

using System.Data.Entity;
using System.Data.Entity.Database;
using System.Data.Entity.Design;
using System.Data.Entity.Infrastructure;
using System.Data.Metadata.Edm;
using System.Data.Objects;
using System.Globalization;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Xml;

public class DontDropDbJustCreateTablesIfModelChanged<T>
                    : IDatabaseInitializer<T> where T : DbContext
{
    private EdmMetadata _edmMetaData;

    public void InitializeDatabase(T context)
    {
        ObjectContext objectContext =
                ((IObjectContextAdapter)context).ObjectContext;

        string modelHash = GetModelHash(objectContext);

        if (CompatibleWithModel(modelHash, context, objectContext))
            return;

        DeleteExistingTables(objectContext);
        CreateTables(objectContext);

        SaveModelHashToDatabase(context, modelHash, objectContext);
    }

    private void SaveModelHashToDatabase(T context, string modelHash,
                                            ObjectContext objectContext)
    {
        if (_edmMetaData != null) objectContext.Detach(_edmMetaData);

        _edmMetaData = new EdmMetadata();
        context.Set<EdmMetadata>().Add(_edmMetaData);

        _edmMetaData.ModelHash = modelHash;
        context.SaveChanges();
    }

    private void CreateTables(ObjectContext objectContext)
    {
        string dataBaseCreateScript =
            objectContext.CreateDatabaseScript();
        objectContext.ExecuteStoreCommand(dataBaseCreateScript);
    }

    private void DeleteExistingTables(ObjectContext objectContext)
    {
        objectContext.ExecuteStoreCommand(Dropallconstraintsscript);
        objectContext.ExecuteStoreCommand(Deletealltablesscript);
    }

    private string GetModelHash(ObjectContext context)
    {
        var csdlXmlString = GetCsdlXmlString(context).ToString();
        return ComputeSha256Hash(csdlXmlString);
    }

    private bool CompatibleWithModel(string modelHash, DbContext context,
                                        ObjectContext objectContext)
    {
        var isEdmMetaDataInStore =
            objectContext.ExecuteStoreQuery<int>(LookupEdmMetaDataTable)
            .FirstOrDefault();

        if (isEdmMetaDataInStore == 1)
        {
            _edmMetaData = context.Set<EdmMetadata>().FirstOrDefault();
            if (_edmMetaData != null)
            {
                return modelHash == _edmMetaData.ModelHash;
            }
        }
        return false;
    }

    private string GetCsdlXmlString(ObjectContext context)
    {
        if (context != null)
        {
            var entityContainerList = context.MetadataWorkspace
                .GetItems<EntityContainer>(DataSpace.SSpace);

            if (entityContainerList != null)
            {
                var entityContainer = entityContainerList.FirstOrDefault();
                var generator =
                    new EntityModelSchemaGenerator(entityContainer);
                var stringBuilder = new StringBuilder();
                var xmlWRiter = XmlWriter.Create(stringBuilder);
                generator.GenerateMetadata();
                generator.WriteModelSchema(xmlWRiter);
                xmlWRiter.Flush();
                return stringBuilder.ToString();
            }
        }
        return string.Empty;
    }

    private static string ComputeSha256Hash(string input)
    {
        byte[] buffer = new SHA256Managed()
            .ComputeHash(Encoding.ASCII.GetBytes(input));

        var builder = new StringBuilder(buffer.Length * 2);
        foreach (byte num in buffer)
        {
            builder.Append(num.ToString("X2",
                CultureInfo.InvariantCulture));
        }
        return builder.ToString();
    }

    private const string Dropallconstraintsscript =
        @"select  
            'ALTER TABLE ' + so.table_name + ' DROP CONSTRAINT ' 
            + so.constraint_name  
            from INFORMATION_SCHEMA.TABLE_CONSTRAINTS so";

    private const string Deletealltablesscript =
        @"declare @cmd varchar(4000)
            declare cmds cursor for 
            Select
                'drop table [' + Table_Name + ']'
            From
                INFORMATION_SCHEMA.TABLES
 
            open cmds
            while 1=1
            begin
                fetch cmds into @cmd
                if @@fetch_status != 0 break
                print @cmd
                exec(@cmd)
            end
            close cmds
            deallocate cmds";

    private const string LookupEdmMetaDataTable =
        @"Select COUNT(*) 
            FROM INFORMATION_SCHEMA.TABLES T 
            Where T.TABLE_NAME = 'EdmMetaData'";
}