Re: How to use a cross column exclude constraint

On Wednesday, August 24, 2022, <awolchute(at)tutanota(dot)com> wrote:

> Hi,
>
> Thank you for the insight!
>
> How would you go about modeling my problem correctly?
>
> The domain constraints are:
> - there are many "records"
> - there are 1:1 links between "records", and the links (table) contain a
> lot of information about the link (so adding a record_id (fk) to the
> records table would also add a ton of columns).
> - the links are bidirectional
> - each "record" can be linked with exactly one "record", so a record
> linking to another does not allow the record being referenced to be in any
> other link either.
> - a graph of records and their connections (links) must be efficiently
> queried / formed
>

Node: [node_id PK, edge_id {FK edge.edge_id}, {Unique: node_id, edge_id)]
Edge: [edge_id PK, …]
Node-Edge: [(node_id, edge_id) {FK node.node_id, node.edge_id}, slot {check
slot in (1,2); not null}, {PK: (edge_id, slot)}, {Unique: node_id}]

That doesn’t enforce “not zero” or missing records, which is possible but
generally a pain, but does enforce that a node may have at most one edge,
and each edge has at most two nodes.

With a deferred not null constraint on node.esge_id I think you can solve
prevent missing links problem, assuming you always add nodes in pairs.
You’d do so ething similar with edge.edge_id if you wanted to avoid
dangling edges (edges without nodes).

David J.