实体框架 CTP 4 - 代码优先自定义数据库初始值设定项

我想实现自定义数据库初始化策略，以便我可以生成数据库架构并使用提供的用户 ID 和密码将其应用到现有的空 SQL 数据库。

不幸的是，内置策略无法提供我正在寻找的内容：

// The default strategy creates the DB only if it doesn't exist - but it does 
// exist so this does nothing
Database.SetInitializer(new CreateDatabaseOnlyIfNotExists<DataContext>());

// Drops and re-creates the database but then this breaks my security mapping and 
// only works if using a “Trusted" connection
Database.SetInitializer(new RecreateDatabaseIfModelChanges<DataContext>());

// Strategy for always recreating the DB every time the app is run. – no good for 
// what I want
Database.SetInitializer(new AlwaysRecreateDatabase<DataContext>());

我已经解决了以下问题，但这不会创建 ModelHash，所以我无法使用“上下文.Database.ModelMatchesDatabase()”验证数据库模式是否已创建并防止多次初始化：

public class Initializer : IDatabaseInitializer<DataContext>  
{ 
    Public void InitializeDatabase(DataContext context)  
    {       
         // this generates the SQL script from my POCO Classes
         var sql = context.ObjectContext.CreateDatabaseScript();

         // As expected - when run the second time it bombs out here with "there is already an
         // object named xxxxx in the database"
         context.ObjectContext.ExecuteStoreCommand(sql); 

         this.seed(context)
         context.SaveChanges();
    }
}  

问题：

有谁知道我如何获取/创建模型哈希？ （这是一个 EdmMetadata 实体）

-Or-

一般而言，使用 Code First CTP 是否有更好的方法来执行此操作？

我遇到了同样的问题。我并没有真正解决它，但我设法运行了一些令人讨厌的解决方法，因此我可以将我的解决方案部署到 AppHarbor ；）

它是一个 IDatabaseInitializer 实现，不会删除数据库，而只是删除所有约束和表，然后使用 ObjectContext.CreateDatabaseScript() 方法生成 sql，然后我将其作为存储命令执行。很像问题中的上述实现。

但我还添加了从模型创建哈希并将其保存在数据库中的功能，当它再次运行时，它会检查当前模型哈希是否与数据库中的哈希相匹配。就像真正的代码优先实现一样。

我无法让它与 context.Database.CompatibleWithModel(true) 中的构建一起工作 - 但这应该也能正常工作，并且将其视为临时解决方法，应该没问题。

using System;
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.Security.Cryptography;
using System.Text;
using System.Xml;
using System.Linq;

namespace Devtalk
{
    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)
                {
                    EntityContainer 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'";
    }
}