RELATED() not working properly, despite Many to 1 relationship

[EnsFlynn]

I'm trying to create a column in a table that is based on the value of two other columns. In both tables I have created a column that is just the two columns concatenated so as to make a composite key I can use for a Many:1 relationship. I have set up the relationship successfully and it is active as a Many:1, yet when I try to use RELATED in a new column for the Many table, it says that it either doesn't exist or has no relationship to any table in the context

NewTargets is the 1 table in the 1:Many, and the related by a column CompKey. I'm completely stumped as to why the RELATED function doesn't work here

[EnsFlynn · Accepted solution]

Ok well I managed to fix my problem soon after posting this. For some reason I was under the impression that RELATED required a 1:many or Many:1 relationship, when it actually needs a 1:1 relationship. 1:Many relationships should be handles by RELATEDTABLE and used as input tables for expressions (in my case I used MAXX).