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From "Avia Efrat (JIRA)" <>
Subject [jira] [Resolved] (ARIA-180) Convert many-to-many for parameter models to one-to-many
Date Sun, 04 Jun 2017 13:02:04 GMT


Avia Efrat resolved ARIA-180.
       Resolution: Fixed
    Fix Version/s: 0.1.0

> Convert many-to-many for parameter models to one-to-many
> --------------------------------------------------------
>                 Key: ARIA-180
>                 URL:
>             Project: AriaTosca
>          Issue Type: Task
>            Reporter: Tal Liron
>            Assignee: Avia Efrat
>            Priority: Minor
>             Fix For: 0.1.0
> We must first discuss this as a team to see if we agree that this is the best solution.
(There was an early discussion between Tal and Maxim.)
> First let's point out that one-to-many is a special case of many-to-many, so that everything
works fine now and continue to work fine.
> However, logically our code right now treats them as one-to-many: there is no case in
which a {{Parameter}} model belongs to more than one model. Parameters are always copied to
the new model, for example during instantiation, or during task creation.
> There are cons to using many-to-many in our case:
> * We generate lots of extra secondary tables, one for each potential relationship
> * Crawling back from a {{Parameter}} to its containing model is quite costly, as it involves
a new SQL query to check for each possible relationship
> * Logical confusion: if we do not write our code to support one parameter belonging to
one model, and yet a user can create such a relationship, what would happen?
> * Slower
> The one advantage of many-to-many is that we *could* potentially optimize in some cases
where the parameter has an identical value and we know would never change, and thus could
safely link it multiple times instead of copying it. This optimization, however, requires
us to be 100% sure that the parameter is immutable: otherwise, if a user changes it (for example
in a task) it would change for all other containers. The questions are: 1) are we ever sure
of immutability? and 2) is this optimization worth the effort of implementing it? The optimization
would only seem to save some disk space.
> Another advantage is that it's much easier to add new models that use {{Parameter}} by
adding an extra table (many-to-many) rather than adding fk columns to an existing table. To
that there is a simple answer: new models can definitely create many-to-many relationships
to anything else. Using one-to-many for our own models doesn't preclude that. (And we can
even add code that automatically tries to look through such many-to-many relationships in
order to find a container.)
> If we decide to switch to one-to-many, we have two approached:
> * Straightforward: one foreign key in {{Parameter}} per each possible containing relationship.
Pros: naturally supported in SQL, cons: we will have lots of fk columns per row in the {{Parameter}}
table, whereby only one will be non-null.
> * Polymorphic one-to-many (type-and-id joins): {{Parameter}} only has a single general-purpose
fk column and another column specifying the type of the fk (node, interface, task, etc.).
Cons: we would need to investigate how to accomplish this in SQLAlchemy.

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