我如何获取一个TDataset来存储空字符串而不是null？

Question

I have a required field in my database (NOT NULL), but empty strings are allowed.

How do I get a delphi TDataset to work with this? With the required property of the field object set to either true or false it still seems to be trying to store null instead of an empty string.

For info im using a TIBDataset and a TIBStringField.

Answer 1

Normally, you can set the value in the OnBeforePost like this:

if IBDataSet1.FieldByName('OPTION_TEXT').IsNull then
begin
  IBDataset1.FieldByName('OPTION_TEXT').Value = '';
end;

However, TIBStringField has an unpublished property EmptyAsNull which you must set to False . The default value is True . When this feature is enabled, the dataset does you a favor and converts empty strings to NULL :

You can turn it off like this:

if IBDataSet1.FieldByName('OPTION_TEXT').IsNull then
begin
  TIBStringField(IBDataset1.FieldByName('OPTION_TEXT')).EmptyAsNull := False;
  IBDataset1.FieldByName('OPTION_TEXT').Value = '';
end;

Alternatively, you could set the EmptyAsNull property on the string fields in your form's OnCreate if you are using static (design time) fields, or wherever your create your fields.

Answer 2

TField has property for default value, but it is string and unfortunately empty string means that there is no default value, so it doesn't help in your case. But you can catch OnBeforePost event from dataset and check, if field is NULL then set it up with empty string.