New function check_cardinality()

[TSchiefer]

• returns the nature of the relationship between parent_table$pk_col and child_table$fk_col

[krlmlr]

Good idea.

• Do we need an is_cardinality...() for each check_cardinality(), would that remove the need for a verbose argument?

• What would be a return value that works for both humans and machines? A number with a string-valued attribute perhaps?

[krlmlr]

Does this abort the function if not a primary key?

[TSchiefer]

Yes, but this is intended (parent table has to have a primary key; this needs to be adapted as soon as compound keys are supported)

[krlmlr]

Before moving on, let's investigate (in a new branch) if is_cardinality_...() functions will make the implementation easier.

[TSchiefer]

Alright, but would you like to generally avoid the verbose argument, also in check_if_subset() and check_set_equality()? In this case we might need a more basic version of these two functions, which would be called by their counterparts, right? We could then use those basic functions for is_cardinality...().

Is that how you intended this?

[krlmlr]

Yes, the verbose argument won't be necessary then.

I'm not sure why we settled for exceptions, but with the new check_cardinality() function it seems like we really should go for functions that check without throwing exceptions. I'd like to play with that code myself first.

[TSchiefer]

I refactored and removed the verbose-argument

[krlmlr]

Looks good, thanks!