我使用TFDMemTable，我创建了一个数据集并用数据填充了我的表格，然后使用FDMemTable.saveToFile来保存我的数据。
现在问题是，我如何向已保存的数据添加新Field并使用默认值填充所有记录？
我尝试将新的Field添加到FDMemTable，然后加载信息，希望它会使用它FieldName自动填充每个字段，并使用空格填充新字段，但是我得到一个错误说：
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我怎样才能解决这个问题 ？是否有解决方法为现有数据添加具有默认值的新字段？
这是一个测试用例：
// here is a simple example, i have a few fields
FDMemTable1.FieldDefs.Add('ID', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('name', ftString, 30, false);
FDMemTable1.FieldDefs.Add('QualityID', ftInteger, 0, false);
FDMemTable1.CreateDataSet;
// i fill the the table with some value
FDMemTable1.Open;
FDMemTable1.AppendRecord([1, 'Movie1', 1]);
FDMemTable1.AppendRecord([2, 'Movie2', 2]);
FDMemTable1.AppendRecord([3, 'Movie3', 1]);
// after seeing the value on the grid, i push a button and save the table as XML
FDMemTable1.saveToFile('Data.xml');
// now after closing the program and running it again, i want to load the data with a new FieldDef Called Episode with a default value 0
// the table is connected to cxGrid, and the moment i try to load, i get the error i mentioned.
FDMemTable1.FieldDefs.Add('ID', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('name', ftString, 30, false);
// this line is NEW
FDMemTable1.FieldDefs.Add('Episode', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('QualityID', ftInteger, 0, false);
FDMemTable1.CreateDataSet;
// i try to load but i get an error
FDMemTable1.loadFromFile('Data.xml');
更新1（根据维多利亚的回答）：
//All the codes below are executed after the code at the top, after the loadFromFile.
FDMemTable1.ResourceOptions.StoreItems := FDMemTable1.ResourceOptions.StoreItems + [siMeta];
FDMemTable1.Close;
//i tried putting FDMemTable1 as the owner but when i try the line "FDMemTable1.Fields.add" i get an Duplicate error!
// and when i try putting nil, i get access violation ! so i tried putting the owner someother random table and it fixed the problem
fieldDefs := TFieldDefs.Create(someRandomFDMemTable);
fieldDefs.Add('Episodes', ftString, 30, false);
FDMemTable1.Fields.Add(fieldDefs.Items[0].CreateField(FDMemTable1));
FDMemTable1.Open;
你可以看到我有两个问题，
1-我在添加新字段时遇到问题！在我收到错误的地方，我首先尝试使用TFieldDef代替TFieldDefs，但我无法让它发挥作用。
2-是否所有列都是空的并且网格上没有数据
当我尝试强有力地解决问题1时，会发生问题2.
答案 0 :(得分：2)
下面的代码在Delphi Seattle中对我来说很好。它紧密地基于你的，但有一些细微的变化（见评论）和一个重要的补充。
问题是，如果你仔细观察FDMemTable1.loadFromFile，它实际上会清除FieldDefs / Fields，所以首先要重新设置它们，所以添加了Episode字段只是从加载的数据集中省略。
为了避免这种情况，我做的补充是使用临时TFDMemTable，将XML文件加载到其中，然后使用CopyDataSet将其内容复制到FDMemTable1。 FDMemTable1在此过程中保留添加的剧集字段，您当然可以在调用CopyDataSet后添加代码以设置剧集字段数据。
procedure TForm1.AddFieldTest;
var
DataFN : String;
TempMemTable : TFDMemTable;
begin
// Added, to avoid relative path for data file name
DataFN := ExtractFilePath(Application.ExeName) + 'Data.Xml';
FDMemTable1.FieldDefs.Add('ID', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('name', ftString, 30, false);
FDMemTable1.FieldDefs.Add('QualityID', ftInteger, 0, false);
FDMemTable1.CreateDataSet;
// i fill the the table with some value
FDMemTable1.Open;
FDMemTable1.AppendRecord([1, 'Movie1', 1]);
FDMemTable1.AppendRecord([2, 'Movie2', 2]);
FDMemTable1.AppendRecord([3, 'Movie3', 1]);
// after seeing the value on the grid, i push a button and save the table as XML
FDMemTable1.saveToFile(DataFN);
// Added, close the dataset and clear the FieldDefs
// Without the FieldDefs.Clear call, your code produces a "Duplicate field ID" error
FDMemTable1.Close;
FDMemTable1.FieldDefs.Clear;
// now after closing the program and running it again, i want to load the data with a new FieldDef Called Episode with a default value 0
// the table is connected to cxGrid, and the moment i try to load, i get the error i mentioned.
FDMemTable1.FieldDefs.Add('ID', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('name', ftString, 30, false);
// this line is NEW
FDMemTable1.FieldDefs.Add('Episode', ftInteger, 0, false);
FDMemTable1.FieldDefs.Add('QualityID', ftInteger, 0, false);
FDMemTable1.CreateDataSet;
// check the FieldCount and existence of the Episode field
Caption := IntToStr(FDMemTable1.FieldCount);
Assert(FDMemTable1.FindField('Episode') <> Nil);
// Create a temporary TFDMemTable
TempMemTable := TFDMemTable.Create(Nil);
try
// load the data into the temporary TFDMemTable
TempMemTable.loadFromFile(DataFN);
// copy the data from the temporary TFDMemTable into FDMemTable1
FDMemTable1.CopyDataSet(TempMemTable, [coAppend]);
// check the FieldCount and existence of the Episode field
Caption := IntToStr(FDMemTable1.FieldCount);
Assert(FDMemTable1.FindField('Episode') <> Nil);
finally
TempMemTable.Free;
end;
end;
答案 1 :(得分：0)
从文件加载数据集后添加该字段（不是字段定义）（假设 siMeta 包含在StoreItems的ResourceOptions属性中TFDMemTable }}）。请注意，在将字段添加到字段集合时，必须关闭数据集。
因此，在从文件加载后立即关闭数据集，添加字段并再次打开它。