序列包含多个元素C＃实体种子

Question

尝试从CSV实体种子来种子，但是每当我运行更新数据库时都会遇到此错误。如果我删除表并运行更新数据库，则该错误不会显示，但是如果我再次运行更新数据库。

CSV文件

Name,
China,
India,
United States,
Indonesia,
Brazil,
Pakistan,
United Kingdom,
Bangladesh,
Russia,
Japan,
Mexico,
Philippines,
Vietnam,
Ethiopia,
Egypt,
Germany,
Iran,
Turkey,
Democratic Republic of the Congo,
Thailand,
France,
Italy,
Burma,
South Africa,
South Korea,
Colombia,
Spain,
Ukraine,

我的国家模特

namespace Domain
{
    public class Country : BaseModel
    {
        public string Name { get; set; }
    }
}

using System.ComponentModel.DataAnnotations;
namespace Domain
{
    public class BaseModel
    {
        [Key]
        public int Id { get; set; }
    }
}

种子

var assembly = Assembly.GetExecutingAssembly();
const string country = "Service.Migrations.Seed.countries.csv";
            using (var stream = assembly.GetManifestResourceStream(country))
            {
                using (var reader = new StreamReader(stream, Encoding.UTF8))
                {
                    var csvReader = new CsvReader(reader);
                    csvReader.Configuration.Delimiter = ",";
                    csvReader.Configuration.WillThrowOnMissingField = false;
                    var countries = csvReader.GetRecords<Country>().ToArray();
                }
            }

错误

No pending explicit migrations.
Running Seed method.
System.InvalidOperationException: Sequence contains more than one element
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__2[TResult](IEnumerable`1 sequence)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
   at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)
   at System.Data.Entity.Migrations.DbSetMigrationsExtensions.AddOrUpdate[TEntity](DbSet`1 set, IEnumerable`1 identifyingProperties, InternalSet`1 internalSet, TEntity[] entities)
   at System.Data.Entity.Migrations.DbSetMigrationsExtensions.AddOrUpdate[TEntity](IDbSet`1 set, Expression`1 identifierExpression, TEntity[] entities)
   at Service.Migrations.Configuration.Seed(HotelContext context) in C:Hotel+ServiceMigrationsConfiguration.cs:line 55
   at System.Data.Entity.Migrations.DbMigrationsConfiguration`1.OnSeed(DbContext context)
   at System.Data.Entity.Migrations.DbMigrator.SeedDatabase()
   at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.SeedDatabase()
   at System.Data.Entity.Migrations.DbMigrator.Upgrade(IEnumerable`1 pendingMigrations, String targetMigrationId, String lastMigrationId)
   at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.Upgrade(IEnumerable`1 pendingMigrations, String targetMigrationId, String lastMigrationId)
   at System.Data.Entity.Migrations.DbMigrator.UpdateInternal(String targetMigration)
   at System.Data.Entity.Migrations.DbMigrator.<>c__DisplayClassc.<Update>b__b()
   at System.Data.Entity.Migrations.DbMigrator.EnsureDatabaseExists(Action mustSucceedToKeepDatabase)
   at System.Data.Entity.Migrations.Infrastructure.MigratorBase.EnsureDatabaseExists(Action mustSucceedToKeepDatabase)
   at System.Data.Entity.Migrations.DbMigrator.Update(String targetMigration)
   at System.Data.Entity.Migrations.Infrastructure.MigratorBase.Update(String targetMigration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.UpdateRunner.Run()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Update(String targetMigration, Boolean force)
   at System.Data.Entity.Migrations.UpdateDatabaseCommand.<>c__DisplayClass2.<.ctor>b__0()
   at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
Sequence contains more than one element

Answer 1

您的ID是自动提出的id ...

您有两种解决方案可以避免这种情况：

解决方案1：

noundert 在您的数据库中循环，并检查名称是否已经存在。如果情况不再添加。

解决方案2：

设置乡村名称如下唯一：

namespace Domain
{
    public class Country : BaseModel
    {
        [Index(IsUnique = true)]
        public string Name { get; set; }
    }
}

当您尝试添加数据库中已经退出的名称时，这将引发异常。您可以使用 try-catch 来管理错误，并记录异常。

Answer 2

这就是我解决问题的方式。

var assembly = Assembly.GetExecutingAssembly();
const string country = "Service.Migrations.Seed.countries.csv";
            using (var stream = assembly.GetManifestResourceStream(country))
            {
                using (var reader = new StreamReader(stream, Encoding.UTF8))
                {
                    var csvReader = new CsvReader(reader);
                    csvReader.Configuration.Delimiter = ",";
                    csvReader.Configuration.WillThrowOnMissingField = false;
                    var countries = csvReader.GetRecords<Country>().ToArray();
                    foreach (var c in countries)
                    {
                        var check = context.Countries.FirstOrDefault(p => p.Name == c.Name);
                        if (check == null)
                        {
                            context.Countries.Add(c);
                            context.SaveChanges();
                        }
                    }
                }
            }