Delphi: "Invalid field type" error with master/detail datasets

Question

I asked about setting up an in-memory dataset with TClientDataset, and was told I had to use the "Create Data Set" command in the form designer. That works just fine until I try to create a master-detail database relation. If I have a field def of type ftDataSet, then running the Create Data Set command (or calling the method at runtime) gives an "Invalid Field Type" error. I did a few searches and found that other people have had this problem before, but nobody ever seems to come up with an answer. Does anyone know how to fix this?

Answer 1

If you create a field of type ftDataSet, you need to define the dataset, else the type is not valid.

You can define the dataset, by:

• select the field.
• select the ChildDefs property and click the [...] button.
• add the fields.

Now you can create the dataset. Unless one of the children is of type ftDataSet in which case you have to recursively repeat the process.

Answer 2

Instead of putting your detail in a ftDataSet column, just use a unique integer or GUID for the key reference, and place the detail data in a different TClientDataSet and have that key. Like you would in a normal database. Each TClientDataSet representing a different table.

Answer 3

A nasty problem that cost me a lot of time already. Finally I found a workaround, described here (Thanks you guys!):

http://www.delphigroups.info/3/6/171869.html

Summary: The culprit are the field definitions of the master client dataset, i.e., the one that contains the ftDataset fields. As soon as the field definitions of the master client dataset are not empty, the command CreateDataSet fails with the above described error message ("Invalid Field Type").

Workaround: Create master-detail tables as instructed, but make sure the field definitions are empty when doing CreateDataset:

a) Designtime: Before you do "master dataset | RightClick | Create DataSet" in the design form, you have to click on the master dataset, go to object inspector, click on FieldDefs, and delete all the fields in the FieldDefs window. (The very first time you built the master dataset there are none in there anyway, but later there are.) Only then do Create Dataset. (I tried this myself with Delphi 2007; it works.)

b) Runtime: Call [Name of Masterdataset].FieldDefs.Clear before you do [Name of Masterdataset].CreateDataSet. (I did not try this out myself, but it is credibly described in the above named link.)

If this was too complicated, please look in the above mentioned link.