Accept composite primary key in id=

[paarthmadan]

Motivation / Background

This is part of the ongoing effort to support composite primary keys.

A high-volume code path (often in tests) is setting a model's primary key using the accessor method. This PR modifies the API of id= to accept an array in composite primary key contexts.

Implementation

Use branching to either write the value to the single attribute, or values to the list of attributes corresponding to the primary key.

Combining these branches using Array removes some handling on when the @primary_key is nil because Array(nil) resolves to []. A test fails in that scenario, so I've opted to use branching.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[nvasilevski]

👍

[nvasilevski]

Wonder if it would be useful to additionally assert for every part of the pk like

assert_equal(1, book.author_id)
assert_equal(2, book.number)