I would like to save an hierarchical folder structure into a SQL database. The class would like this:
public class Folder
{
public Folder()
{
Children = new List<Folder>();
}
public string Name { get; set; }
public int Id { get; set; }
public int? ParentId { get; set; }
public Folder Parent { get; set; }
public ICollection<Folder> Children { get; set; }
}
I'm trying to map it using Entity Framework Core:
builder.Entity<Folder>()
.HasKey(i => i.Id);
// Relation 1
builder.Entity<Folder>()
.HasMany(e => e.Children)
.WithOne(e => e.Parent)
.HasForeignKey(e => e.ParentId);
// Relation 2
builder.Entity<Folder>()
.HasOne(f => f.Parent)
.WithMany(f => f.Children)
.OnDelete(Microsoft.EntityFrameworkCore.Metadata.DeleteBehavior.Cascade);
If I try to update the database, I get the following exception:
System.Data.SqlClient.SqlException: Introducing FOREIGN KEY constraint 'FK_Folders_Folders_ParentId' on table 'Folders' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint or index. See previous errors.
at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action1 wrapCloseInAction)
1 completion, Boolean sendToPipe, Int32 timeout, Boolean asyncWrite, String methodName)
at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
at System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async, Int32 timeout, Boolean asyncWrite)
at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource
at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String executeMethod, IReadOnlyDictionary2 parameterValues, Boolean openConnection, Boolean closeConnection)
2 parameterValues, Boolean manageConnection)
at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteNonQuery(IRelationalConnection connection, IReadOnlyDictionary
at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationCommandExecutor.ExecuteNonQuery(IEnumerable`1 migrationCommands, IRelationalConnection connection)
at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.Migrate(String targetMigration)
at Microsoft.EntityFrameworkCore.Design.MigrationsOperations.UpdateDatabase(String targetMigration, String contextType)
at Microsoft.EntityFrameworkCore.Tools.Cli.DatabaseUpdateCommand.<>c__DisplayClass0_0.b__0()
at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.Execute(String[] args)
at Microsoft.EntityFrameworkCore.Tools.Cli.Program.Main(String[] args)ClientConnectionId:f0c08167-fba7-4afa-baf0-45909e9a1f4b
Error Number:1785,State:0,Class:16Introducing FOREIGN KEY constraint 'FK_Folders_Folders_ParentId' on table 'Folders' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint or index. See previous errors.
I also tried mapping it without the 'Relation 2', it works but then when I load the items from database they return as single items with the Children property not set.
What is the correct way of storing such kind of data?