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Master-detail relationship

Hi,

I have database app that has a master table with several detail tables. The link is through the a field called "title".

When I edit this the title in the master table, how do I make it so that the change of title is reflected in the detail tables?

At the moment when I change the title in the master table, a new record is added in the detail tables rather than the old record being modified. This causes data associated with the old title to become "lost".

Thanks

John
0
JohnE
Asked:
JohnE
1 Solution
 
efj67Commented:
I'm not sure I understand. Let me get this, you have a relational link with several child tables, but when you change the linking data you want the change to be reflected in the child tables? You will have to do some coding to accomplish this.

1. First you want to capture the linking data as it is presently in a variable.

2. Put the new info in a variable also.

3. Then use a decision statement to change data records in the child tables.

Your code might look something like this

var

oldinfo : <datatype>
newifo : <datatype>

begin
editnewinfofield.text:=newinfo;
oldinfo:=dbeditfield.value;
firstchildtable.open;
firstchildtable.first;
while firstchildtable not eof
 do
  begin
   if firstchildtablefield=oldinfo then
   begin
    firstchildtablefield.vaule:=newinfo;
    firstchildtablefield.post;
   else
    firstchildtablefield.next;
   end;
end;
end.


Note you may want to add some safety features in this code to insure that the integrity of the tables is maintained. If you are planning to network this app, this should be an offline batch operation.

I do realize this is rather bland but it should get the brain cooking.

Feel free to e-mail if I am off target.
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kretzschmarCommented:
for what database you need this?
0
 
JohnEAuthor Commented:
Initially it was for paradox tables, but I have migrated to DBISAM.
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kretzschmarCommented:
hi johne,

i am not familiar with dbisam, therefore i don't know,
if there could be done a constraint for modify forwarding at databaselevel.

in this case, you can(must) it do in the way, which  efj67 suggested, also if there are errors ( efj67 you've forgotten to edit the detailtable)

meikl
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JohnEAuthor Commented:
Hi,

Below is some code based on what you suggested. This works, but only for the first instance of the title occuring in the secondary table.

If the title occurs more than once, only one instance is converted to the new title.

How can I make all instances change?

Table 1{master table}
Table 2{detail table}
Table 3{same table as 2, but without m/d link}

Thanks

John

procedure TForm1.UpdateClick(Sender:
TObject);
var
   oldinfo : string;
   newinfo : string;
begin
     Table1.Edit;
     Table2.close;
     Table1.post;
     newinfo := DBEdit1.text;
     oldinfo:= Edit1.Text;
with Table3 do      
begin
          first;
          open;
          edit;
          FindKey([Table1.FieldbyName('Title').AsString]);
          begin
          while not Eof do
            begin
                if Fields[0].AsString =oldinfo then
                   begin
                       Fields[0].AsString:=newinfo;
                       post ;
                       next;
                    end
            else
                next;
                Edit;
            end;
          end;
    end;
   Table2.open;
end;
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JohnEAuthor Commented:
Hi,

Below is some code based on what you suggested. This works, but only for the first instance of the title occuring in the secondary table.

If the title occurs more than once, only one instance is converted to the new title.

How can I make all instances change?

Table 1{master table}
Table 2{detail table}
Table 3{same table as 2, but without m/d link}

Thanks

John

procedure TForm1.UpdateClick(Sender:
TObject);
var
   oldinfo : string;
   newinfo : string;
begin
     Table1.Edit;
     Table2.close;
     Table1.post;
     newinfo := DBEdit1.text;
     oldinfo:= Edit1.Text;
with Table3 do      
begin
          first;
          open;
          edit;
          FindKey([Table1.FieldbyName('Title').AsString]);
          begin
          while not Eof do
            begin
                if Fields[0].AsString =oldinfo then
                   begin
                       Fields[0].AsString:=newinfo;
                       post ;
                       next;
                    end
            else
                next;
                Edit;
            end;
          end;
    end;
   Table2.open;
end;
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kretzschmarCommented:
hi johne,

did you thought about to use a query
(if possible with dbisam)

the statement is like

Update YourDetailTableName Set YourFieldName = YourNewValue Where YourFieldName = YourOldValue

and fire the statement with the query.ExecSQL-method

its much less code(if parametewrs are used) and its faster than iterating through the table

meikl
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floriangrimmCommented:
thats really good idea!!!
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JohnEAuthor Commented:
Meikl,

Your SQL suggestion worked perfectly; and you're right - half the code and heaps faster.

How do I give you the points?

Thanks for your help.

John
0
 
kretzschmarCommented:
hi johne,

>How do I give you the points?
you've just done it

glad that you get it work
thanks for accepting my comment as answer
good luck again

meikl ;-)
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