added instance check in query, create and destroy

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (c78839d) 90.86% compared to head (8429ae5) 90.72%.

Files	Patch %	Lines
src/views/api.py	33.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #238 +/- ## ========================================== - Coverage 90.86% 90.72% -0.14% ========================================== Files 29 29 Lines 2506 2512 +6 ========================================== + Hits 2277 2279 +2 - Misses 229 233 +4 ``` | [Flag](https://app.codecov.io/gh/jdabtieu/CTFOJ/pull/238/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonathan+Wu) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jdabtieu/CTFOJ/pull/238/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonathan+Wu) | `90.72% <33.33%> (-0.14%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonathan+Wu#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jdabtieu]

Thanks! Merged into master.