我只是能够与以下考虑这样做在EF 4.1:
的数据库仍然被删除并重新创建 - 它必须是模式才能反映您的模型更改 - 但您的数据保持不变。
以下是您将数据库读入内存中的POCO对象,然后在POCO对象成功将其写入内存之后,您可以让EF删除并重新创建数据库。下面是一个例子
public class NorthwindDbContextInitializer : DropCreateDatabaseAlways<NorthindDbContext> {
/// <summary>
/// Connection from which to ead the data from, to insert into the new database.
/// Not the same connection instance as the DbContext, but may have the same connection string.
/// </summary>
DbConnection connection;
Dictionary<Tuple<PropertyInfo,Type>, System.Collections.IEnumerable> map;
public NorthwindDbContextInitializer(DbConnection connection, Dictionary<Tuple<PropertyInfo, Type>, System.Collections.IEnumerable> map = null) {
this.connection = connection;
this.map = map ?? ReadDataIntoMemory();
}
//read data into memory BEFORE database is dropped
Dictionary<Tuple<PropertyInfo, Type>, System.Collections.IEnumerable> ReadDataIntoMemory() {
Dictionary<Tuple<PropertyInfo,Type>, System.Collections.IEnumerable> map = new Dictionary<Tuple<PropertyInfo,Type>,System.Collections.IEnumerable>();
switch (connection.State) {
case System.Data.ConnectionState.Closed:
connection.Open();
break;
}
using (this.connection) {
var metaquery = from p in typeof(NorthindDbContext).GetProperties().Where(p => p.PropertyType.IsGenericType)
let elementType = p.PropertyType.GetGenericArguments()[0]
let dbsetType = typeof(DbSet<>).MakeGenericType(elementType)
where dbsetType.IsAssignableFrom(p.PropertyType)
select new Tuple<PropertyInfo, Type>(p, elementType);
foreach (var tuple in metaquery) {
map.Add(tuple, ExecuteReader(tuple));
}
this.connection.Close();
Database.Delete(this.connection);//call explicitly or else if you let the framework do this implicitly, it will complain the connection is in use.
}
return map;
}
protected override void Seed(NorthindDbContext context) {
foreach (var keyvalue in this.map) {
foreach (var obj in (System.Collections.IEnumerable)keyvalue.Value) {
PropertyInfo p = keyvalue.Key.Item1;
dynamic dbset = p.GetValue(context, null);
dbset.Add(((dynamic)obj));
}
}
context.SaveChanges();
base.Seed(context);
}
System.Collections.IEnumerable ExecuteReader(Tuple<PropertyInfo, Type> tuple) {
DbCommand cmd = this.connection.CreateCommand();
cmd.CommandText = string.Format("select * from [dbo].[{0}]", tuple.Item2.Name);
DbDataReader reader = cmd.ExecuteReader();
using (reader) {
ConstructorInfo ctor = typeof(Test.ObjectReader<>).MakeGenericType(tuple.Item2)
.GetConstructors()[0];
ParameterExpression p = Expression.Parameter(typeof(DbDataReader));
LambdaExpression newlambda = Expression.Lambda(Expression.New(ctor, p), p);
System.Collections.IEnumerable objreader = (System.Collections.IEnumerable)newlambda.Compile().DynamicInvoke(reader);
MethodCallExpression toArray = Expression.Call(typeof(Enumerable),
"ToArray",
new Type[] { tuple.Item2 },
Expression.Constant(objreader));
LambdaExpression lambda = Expression.Lambda(toArray, Expression.Parameter(typeof(IEnumerable<>).MakeGenericType(tuple.Item2)));
var array = (System.Collections.IEnumerable)lambda.Compile().DynamicInvoke(new object[] { objreader });
return array;
}
}
}
这个例子依赖于一个ObjectReader类,你可以找到here如果你需要它。
我不会打扰博客文章,阅读documentation。
最后,我仍然建议您在运行初始化之前始终备份数据库。 (例如,如果Seed方法抛出异常,则所有数据都在内存中,因此一旦程序终止,您可能会丢失数据。)模型更改不完全是事后的行为,因此请务必将数据备份。
Alpha 3现已发布:http://blogs.msdn.com/b/adonet/archive/2011/09/21/code-first-migrations-alpha-3-no-magic-walkthrough.aspx – 2011-10-12 22:58:03