fetching 'KEY' error

[davidgodinez]

Bug Report

Description

When making a fetch, using the syntax table_name.fetchn('KEY') brings about an attribute error.

Reproducibility

Include:

• OS MACOS BIG SUR 11.5.2
• MATLAB Version: MATLAB_R2021b
• DataJoint Version: version 3.4.3
• Complete error stack as a result of evaluating the above steps:

  
  Error using dj.internal.Header/project (line 199)
  Attribute `KEY` does not exist

Error in dj.internal.GeneralRelvar/compile (line 682) header.project(self.operands(2:end));

Error in dj.internal.GeneralRelvar/fetch (line 212) [hdr, sql_] = self.compile;

Error in dj.internal.GeneralRelvar/fetchn (line 285) s = self.fetch(varargin{:});

### Expected Behavior
The expected behavior is that the key would be fetched. 

### Screenshots
<img width="1675" alt="Screen Shot 2022-02-02 at 7 24 08 PM" src="https://user-images.githubusercontent.com/43123708/152376781-b2dba0ef-d2ca-476c-9416-3ce8682d05ae.png">
<img width="822" alt="Screen Shot 2022-02-03 at 9 45 59 AM" src="https://user-images.githubusercontent.com/43123708/152376890-20603c83-320d-459f-bee7-ae0e510f7975.png">

[dimitri-yatsenko]

dataJoint-MATLAB does not have "KEY" implemented. To get the key from table q, one just omits all arguments in fetch: q.fetch()

[davidgodinez]

Should we change our documentation here in that case?