Lookup stage's Key Expression is greyed out

[Hongqing]

I use a Lookup stage to lookup from a Lookup File Set Stage. When I opened up the Lookup Stage, I found that the "Key Expression" for the key field of the lookup file set is greyed out. It's not giving me any error msg. and the job passed the compile. Why is it greyed out? Do not we have to explicitly specify the lookup join?

[ray.wurlod]

Have you declared any column to be the key? The key expression is only enabled when you have a key column selected.

[Hongqing]

Yes, I have declared a key in the LookupFileSet and a key in the main flow stream. I can see the key field is bold, but the the key exression next to it was greyed out. When I right click there, the option "create key expression" or something like that was also greyed out. I still can not figure out why? Thanks in advance.

Have you declared any column to be the key? The key expression is only enabled when you have a key column selected.

[Eric]

My understanding ...
You have to definie the Key when you create the Lookup FileSet.
This schema info is stored within the FileSet and thus can not be changed without re-creating the FileSet.
When the job runs (maybe for compile too?) the Key (schema info) is read from the Lookup FileSet rather than reading the info from the job design.

[Hongqing]

I have done all of that, however, we still need to define a join in the lookup stage to specify the join in the lookup on the keys defined in the lookup file set and the main stream, right? I could not create this key expression there. Help please!

My understanding ...
You have to definie the Key when you create the Lookup FileSet.
This schema info is stored within the FileSet and thus can not be changed without re-creating the FileSet.
When the job runs (maybe for compile too?) the Key (schema info) is read from the Lookup FileSet rather than reading the info from the job design.