let is used to temporarily assign a value to a variable in an EAL expression.

derived associations are used to create "shortcuts" in your model or to split complex navigations into parts.

Combining these two can lead to unexpected results.

Example with single link association

If you have a derived single link association called lastSubPart, derived like

self.subParts->last

then you have a method with this content

let lp = self.lastSubPart in 
(
  self.subParts.add(newPart);
  newPart.Name = 'copy of ' + lp.Name
)

You might expect newPart.Name to hold 'copy of <name of the lastsubpart>', but it doesn't.

That's because lp is NOT holding the object, it's holding a subscription to the derivation to the object.

When the code does self.subParts.add(newPart), lp changes to point to the new object.

I.e. every time you use lp, the derived single link association is reevaluated.

let lp = self.lastSubPart->first in 
(
  self.subParts.add(newPart);
  newPart.Name = 'copy of ' + lp.Name
)

The first operator here converts the object to a set and then takes the first, which is the object itself, not the subscription.