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A completely central feature of creating Source- and Target Maps in Studio is that you have to assign values to the fields the Map will output
In this article, I will use the term target to cover both the Target Interface imported into the Source Map and the Target System Metadata imported into the Target Map.
Field Calculation Order
Initially the fields demanding values are listed in the order they are defined by the target. The order of the fields is important, because this is the order in which the resulting, generated Engine will calculate and assign the value for each field. Sometimes, however, you may want to modify the order in which field values are calculated and assigned.
The value assigned to a given field may be needed to calculate and assign the value to another field. For instance as an input parameter for a rule calculating the value of the other field. In this case, the order of calculation is important. The first field must appear in the list prior to the second field, in order for its value to be assigned first, so it can be used to calculate and assign the value of the second field. As a side note, the Studio validation will catch if a field value is used before its value has been assigned.
But there is no guarantee that the order of the fields from the target appears in the desired calculation order. In this case, Studio comes to your aid by allowing you to change the order of the fields in the list and thereby changing the order of calculation. Use the Move up/Move down buttons to move a field in the list and the Reset button to reset the field order to the order they appear in the target:
A new target may of course be imported at any time. With an import, new fields may well appear in the list of fields. If the list has not been reordered, any new field will appear in the position given by the imported target - the list will keep on being in the order corresponding to the target. But if the list has been reordered, any new fields will be added to the end of the list. You can always use the Reset button to remove any manual field ordering and reset the list to the order defined by the target.
Field Output Order
Regardless of the calculation order, the fields will always be output by the generated Engine in the order defined by the target.