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Re: [ddlm-group] Adding a DDLm attribute for uniqueness

For what it is worth, I am also content with the current DDL2 and DDLm approach as you will see from my response to the original issue on Github.  John - the idea is that these are alternate candidate keys.

The proponent is aware of the currently available attributes for category keys. I believe this proposal is aimed at providing further checks in software for data names that are not category keys but are also supposed to be unique, the canonical example being symmetry operators. My objection is that expansion dictionaries can remove this uniqueness, e.g. listing magnetic symmetry operations as spatial symmetry operations + magnetic symmetry operations might involve repeating symmetry operations.  We have developed an approach in DDLm to handle this for expanding category keys (the _audit.schema data name) but dealing with this for an independent uniqueness attribute seems to be a bit messy and I don't really see the benefit of that extra definitional work.

The other thing I've pointed out is that ad-hoc uniqueness checks can be coded in dREL and placed in a dictionary of data names to be used for validation.

I think it would be good for Antanas (the proponent in this case) to join this group as he has shown a lot of interest in DDLm and has already developed some good tools to leverage DDLm dictionaries, so I have asked Brian to add him in.

On Fri, 28 Jun 2019 at 10:46, Herbert J. Bernstein <yayahjb@gmail.com> wrote:
The troubling part of this is "unique within a loop".   The handling of relational keys is complex but clear, because
categories are well-defined.  The content of a loop beyond the relational model is not clear without much more information,
especially for numeric data and unicode data, both of which come with major ambiguities in terms of uniqueness.  The
situation gets even more confusing when trying to make a database from multiple entries.  We add keys precisely to allow for duplication
of existing keys.  How will we handle these new pseudo-keys?  I would suggest that any proposal be presented with
a clear view of how we will handle databases without breaking the new proposed constraints

On Thu, Jun 27, 2019 at 7:36 PM James Hester <jamesrhester@gmail.com> wrote:
Dear DDLm group,

A proposal has been put forward to enhance DDLm by the addition of an attribute that specifies that values of an item or group of items must be unique within a loop. This is something that was present in DDL1 and is absent in DDL2 and DDLm. The details of the proposal and some initial discussion are available at https://github.com/COMCIFS/cif_core/issues/119 .

Please provide your thoughts on this proposal.

best wishes,
James.
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