How to create fields programatically?
lazarus / 16-Databases

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TurboRascal · 2012-09-01 17:58

I wanted to try something from a previous post here, but I chose to do
it in a simple command line program and I've faced a problem I don't
have in a Lazarus-managed GUI programs. I have to create all objects
manually, but I've find out I have no idea how to create a TField
instance!

I've tried this according to the FCL documentation:
Code: [Select]DataSet.Fields.Add(Field);
Also this, simply using the constructor, as usual:
Code: [Select]Field := TField.Create(nil);
Whatever I did, either I got an access violation (SIGSEGV on Linux) or
if I did not and then tried using the field in the dataset's Fields[n]
property, I got an "index out of bounds" error even with the index
being zero.

I've checke the documentation, FCL docs do have TFields documented,
but it is very brief and nothing of it works in this example. The only
information I found on the wiki, in the documentation about databases,
is a redlink with a promise there will be explained how to
programatically create fields, but so far nobody has written it.

So to recap once again, nothing that works in a GUI Lazarus app works
here, and also I couldn't do it properly in the way objecs are usally
managed.  So, obviously there is another step required, or is
something done differently . Could somebody explain how is it done?
Then perhaps we could finally complete the above mentioned wiki
documentation

EDIT: The dataset used was TMemDataset, so it is possible it affects
the required steps...

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taazz · 2012-09-01 23:39

Code: [Select]  MemDataset1.FieldDefs.Add('FirstName',ftstring, 40);
  MemDataset1.FieldDefs.Add('LastName',ftstring, 60);
  MemDataset1.FieldDefs.Add('HomePhone',ftstring, 20);
  MemDataset1.CreateTable;

In short you create field definitions and the fields are created for
you when the dataset is activated or created.

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TurboRascal · 2012-09-03 11:51

Quote from: taazz on September 02, 2012, 01:39:17 amCode: [Select] 
MemDataset1.FieldDefs.Add('FirstName',ftstring, 40);
  MemDataset1.FieldDefs.Add('LastName',ftstring, 60);
  MemDataset1.FieldDefs.Add('HomePhone',ftstring, 20);
  MemDataset1.CreateTable;

In short you create field definitions and the fields are created for
you when the dataset is activated or created.

Thank you, this works fine without using TField. Obviously the
FieldDefs does that which was previously automatically done for me. I
do get an error, but perhaps it's a bug in my svn snapshot. I'll check
with a fresh one and then with 1.0 if it fails, to make sure what the
problem is.

I'm still not sure how to properly initialize and use TField from
scratch though... Any hints?

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taazz · 2012-09-03 12:09

The following is the CreateField as found in the TFieldDef class
(removed the debug staff for clarity)
QuoteCode: [Select]Function TFieldDef.CreateField(AOwner: TComponent):
TField;

Var TheField : TFieldClass;

begin
  TheField:=GetFieldClass;
  if TheField=Nil then
    DatabaseErrorFmt(SUnknownFieldType,[FName]);
  Result:=Thefield.Create(AOwner);
  Try
    Result.Size:=FSize;
    Result.Required:=FRequired;
    Result.FFieldName:=FName;
    Result.FDisplayLabel:=DisplayName;
    Result.FFieldNo:=Self.FieldNo;
    Result.SetFieldType(DataType);
    Result.FReadOnly:= (faReadOnly in Attributes);
    Result.Dataset:=TFieldDefs(Collection).Dataset;
    If (Result is TFloatField) then
      TFloatField(Result).Precision:=FPrecision;
    if (Result is TBCDField) then
      TBCDField(Result).Precision:=FPrecision;
    if (Result is TFmtBCDField) then
      TFmtBCDField(Result).Precision:=FPrecision;
  except
    Result.Free;
    Raise;
  end;

end;

As far as I can see you need to set all field's properties especially
fieldname, size etc before you set the field's dataset and add it to
the dataset's fields collection before the dataset is
opened/activated.

Regards

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TurboRascal · 2012-09-03 15:12

Great, thanks! I'll add this to the wiki to resolve the red link