我已在我的实体框架项目上启用了代码优先迁移，并添加了一些迁移，这些迁移执行重命名表等操作。但是，我现在已经删除了数据库并导致实体框架根据我的最新数据模型生成一个全新的数据库。如果我试着跑：
PM> Add-Migration TestMigration
...它告诉我，我需要先应用现有的迁移。所以我跑：
PM> Update-Database
...但问题是它正在尝试更新不需要更新的数据库;它已经基于最新的数据模型。因此，当我尝试重命名现在不存在的表时，我收到错误。
我是否可以通过某种方式向数据迁移表明我的数据库是最新的，并且不需要在其上运行任何迁移？我该怎么办？
答案 0 :(得分：9)
我找到了一种方法来表明我的数据库是最新的，并且（不出所料）基于修改__MigrationHistory表，这是代码优先的迁移用于确定要应用于哪些迁移的表运行Update-Database时的数据库。
顺便说一句，在研究这个答案时，我遇到了一个非常好的代码优先迁移命令参考，可以在以下位置找到：http://dotnet.dzone.com/articles/ef-migrations-command
当EF自动从头创建数据库时，它将始终将一个条目放入__MigrationHistory表，该条目将具有MigrationId (currentDateTime)_InitialCreate。这表示EF刚刚执行的数据库的初始创建。但是，您的迁移历史记录不会以MigrationId开头，因为您将开始使用其他内容。
为了“欺骗”代码优先迁移，认为您正在进行最新迁移，您需要从新创建的数据库的(currentDateTime)_InitialCreate表中删除该__MigrationHistory条目，然后插入如果您仍然拥有已应用迁移的旧数据库，那会是什么。
因此，首先从新生成的DB的__MigrationHistory表中删除所有内容。然后，进入包管理器控制台并运行：
PM> Update-Database -Script
从生成的SQL脚本中，拉出以：
开头的所有行INSERT INTO [__MigrationHistory]...
然后，在新创建的数据库的上下文中运行这些INSERT语句。检查__MigrationHistory表中是否存在这些行中的每一行。下次运行时：
PM> Update-Database
...您应该收到一条消息“没有待处理的基于代码的迁移”。恭喜 - 您已经愚弄了代码优先迁移，认为您现在正在进行最新迁移，并且您可以继续从此处继续添加新迁移。
我认为应该有一些自动化的方法来构建到EF代码中 - 尽管......可能他们应该添加类似的内容：
PM> Update-Database -MigrationsTableOnly
...它会破坏迁移表中的现有条目，只需将新条目插入到项目中定义的每个迁移的迁移历史记录中，但实际上不会尝试运行迁移。好吧。
<强>更新
我已经找到了一种使用自定义初始化程序的Seed方法很好地自动化的方法。基本上，Seed方法会在创建数据库时删除现有的迁移历史记录数据，并插入迁移历史记录。在我的数据库上下文构造函数中，我注册了自定义初始化程序，如下所示：
public class MyDatabaseContext : DbContext {
public MyDatabaseContext() : base() {
Database.SetInitializer(new MyDatabaseContextMigrationHistoryInitializer());
}
自定义初始化程序本身如下所示：
/// <summary>
/// This initializer clears the __MigrationHistory table contents created by EF code-first when it first
/// generates the database, and inserts all the migration history entries for migrations that have been
/// created in this project, indicating to EF code-first data migrations that the database is
/// "up-to-date" and that no migrations need to be run when "Update-Database" is run, because we're
/// already at the latest schema by virtue of the fact that the database has just been created from
/// scratch using the latest schema.
///
/// The Seed method needs to be updated each time a new migration is added with "Add-Migration". In
/// the package manager console, run "Update-Database -Script", and in the SQL script which is generated,
/// find the INSERT statement that inserts the row for that new migration into the __MigrationHistory
/// table. Add that INSERT statement as a new "ExecuteSqlCommand" to the end of the Seed method.
/// </summary>
public class MyDatabaseContextMigrationHistoryInitializer : CreateDatabaseIfNotExists<MyDatabaseContext> {
/// <summary>
/// Sets up this context's migration history with the entries for all migrations that have been created in this project.
/// </summary>
/// <param name="context">The context of the database in which the seed code is to be executed.</param>
protected override void Seed(MyDatabaseContext context) {
// Delete existing content from migration history table, and insert our project's migrations
context.Database.ExecuteSqlCommand("DELETE FROM __MigrationHistory");
context.Database.ExecuteSqlCommand("INSERT INTO __MigrationHistory (MigrationId, Model, ProductVersion) VALUES ('201210091606260_InitialCreate', 0x1F8B0800000000000400ECBD07601C499625262F6DCA7B7F4AF54AD7E074A..., '5.0.0.net40')");
context.Database.ExecuteSqlCommand("INSERT INTO __MigrationHistory (MigrationId, Model, ProductVersion) VALUES ('201210102218467_MakeConceptUserIdNullable', 0x1F8B0800000000000400ECBD07601C499625262F6DCA7B7F4..., '5.0.0.net40')");
context.Database.ExecuteSqlCommand("INSERT INTO __MigrationHistory (MigrationId, Model, ProductVersion) VALUES ('201210231418163_ChangeDateTimesToDateTimeOffsets', 0x1F8B0800000000000400ECBD07601C499625262F6D..., '5.0.0.net40')");
context.Database.ExecuteSqlCommand("INSERT INTO __MigrationHistory (MigrationId, Model, ProductVersion) VALUES ('201210251833252_AddConfigSettings', 0x1F8B0800000000000400ECBD07601C499625262F6DCA7B7F4AF54AD7E..., '5.0.0.net40')");
context.Database.ExecuteSqlCommand("INSERT INTO __MigrationHistory (MigrationId, Model, ProductVersion) VALUES ('201210260822485_RenamingOfSomeEntities', 0x1F8B0800000000000400ECBD07601C499625262F6DCA7B7F4AF5..., '5.0.0.net40')");
}
}
答案 1 :(得分：0)
转到sql server中的MigrationHistory表（在system文件夹下）
它有您的迁移行和数据库哈希值，它必须与迁移文件中的一行相同，只需将其从db复制到文件。
换句话说，您需要将MigrationHistory表与实际迁移同步。
答案 2 :(得分：0)
此实现不需要手动维护要插入__MigrationHistory的记录。迁移由指定的程序集确定。
也许这有帮助。
我要感谢@Jez的初步想法。
/// <summary>
/// An implementation of IDatabaseInitializer that will:
/// 1. recreate database only if the database does not exist
/// 2. actualize __MigrationHistory to match current model state (i.e. latest migration)
/// </summary>
/// <typeparam name="TContext">The type of the context.</typeparam>
public class CreateDatabaseIfNotExistsAndMigrateToLatest<TContext> : CreateDatabaseIfNotExists<TContext>
where TContext : DbContext
{
private readonly Assembly migrationsAssembly;
/// <summary>
/// Gets the migration metadata for types retrieved from <paramref name="assembly"/>. Types must implement <see cref="IMigrationMetadata"/>.
/// </summary>
/// <param name="assembly">The assembly.</param>
/// <returns></returns>
private static IEnumerable<IMigrationMetadata> GetMigrationMetadata(Assembly assembly)
{
var types = assembly.GetTypes().Where(t => typeof(IMigrationMetadata).IsAssignableFrom(t));
var migrationMetadata = new List<IMigrationMetadata>();
foreach (var type in types)
{
migrationMetadata.Add(
(IMigrationMetadata)Activator.CreateInstance(type));
}
return migrationMetadata.OrderBy(m => m.Id);
}
/// <summary>
/// Gets the provider manifest token.
/// </summary>
/// <param name="db">The db.</param>
/// <returns></returns>
private static string GetProviderManifestToken(TContext db)
{
var connection = db.Database.Connection;
var token = DbProviderServices.GetProviderServices(connection).GetProviderManifestToken(connection);
return token;
}
/// <summary>
/// Gets the migration SQL generator. Currently it is <see cref="SqlServerMigrationSqlGenerator"/>.
/// </summary>
/// <returns></returns>
private static MigrationSqlGenerator GetMigrationSqlGenerator()
{
return new SqlServerMigrationSqlGenerator();
}
/// <summary>
/// Creates the operation for inserting into migration history. Operation is created for one <paramref name="migrationMetadatum"/>.
/// </summary>
/// <param name="migrationMetadatum">The migration metadatum.</param>
/// <returns></returns>
private static InsertHistoryOperation CreateInsertHistoryOperation(IMigrationMetadata migrationMetadatum)
{
var model = Convert.FromBase64String(migrationMetadatum.Target);
var op = new InsertHistoryOperation(
"__MigrationHistory",
migrationMetadatum.Id,
model,
null);
return op;
}
/// <summary>
/// Generates the SQL statements for inserting migration into history table.
/// </summary>
/// <param name="generator">The generator.</param>
/// <param name="op">The operation.</param>
/// <param name="token">The token.</param>
/// <returns></returns>
private static IEnumerable<MigrationStatement> GenerateInsertHistoryStatements(
MigrationSqlGenerator generator,
InsertHistoryOperation op,
string token)
{
return generator.Generate(new[] { op }, token);
}
/// <summary>
/// Runs the SQL statements on database specified by <paramref name="db"/> (<see cref="DbContext.Database"/>).
/// </summary>
/// <param name="statements">The statements.</param>
/// <param name="db">The db.</param>
private static void RunSqlStatements(IEnumerable<MigrationStatement> statements, TContext db)
{
foreach (var statement in statements)
{
db.Database.ExecuteSqlCommand(statement.Sql);
}
}
/// <summary>
/// Initializes a new instance of the <see cref="CreateDatabaseIfNotExistsAndMigrateToLatest{TContext}"/> class.
/// </summary>
/// <param name="migrationsAssembly">The migrations assembly.</param>
public CreateDatabaseIfNotExistsAndMigrateToLatest(Assembly migrationsAssembly)
{
this.migrationsAssembly = migrationsAssembly;
}
protected override void Seed(TContext context)
{
base.Seed(context);
// Get migration metadata for migrationAssembly
var migrationMetadata = GetMigrationMetadata(migrationsAssembly);
// Crate DbContext
var db = Activator.CreateInstance<TContext>();
// Remove newly created record in __MigrationHistory
db.Database.ExecuteSqlCommand("DELETE FROM __MigrationHistory");
// Get provider manifest token
var token = GetProviderManifestToken(db);
// Get sql generator
var generator = GetMigrationSqlGenerator();
foreach (var migrationMetadatum in migrationMetadata)
{
// Create history operation
var op = CreateInsertHistoryOperation(migrationMetadatum);
// Generate history insert statements
var statements = GenerateInsertHistoryStatements(generator, op, token);
// Run statements (SQL) over database (db)
RunSqlStatements(statements, db);
}
}
}