序列包含一個以上的元素c＃實體種子

Question

嘗試從CSV實體播種國家/地區，但在我運行Update-Database時遇到此錯誤。 如果我刪除表並運行Update-Database，則不會顯示該錯誤，但是如果我再次運行Update-Database。

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,

我的國家模型

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; }
    }
}

種子

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();
                }
            }

錯誤

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+\Service\Migrations\Configuration.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是自動遞增的，因此如果您不檢查當前的國家/地區名稱是否已存在於數據庫中，EF將使用新的ID創建一個新條目...

您有兩種解決方案可以避免這種情況：

解決方案1：

在數據庫中的國家/地區內循環，然后檢查名稱是否已存在。 如果是這種情況，請勿再次添加。

解決方案2：

像這樣設置國家名稱為唯一：

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

當您嘗試添加數據庫中已經存在的名稱時，這將引發異常。 您可以使用try-catch來管理錯誤並記錄異常。

Answer 2

這就是我解決問題的方式。

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();
                        }
                    }
                }
            }