[fix:ux] Fixed rendering of JSON fields in readonly mode #848

[pandafy]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Fixes #848

Screenshot

[coveralls]

coverage: 98.78% (+0.02%) from 98.758%
when pulling 27f544f on issues/848-readonly-config-field
into 6ae8103 on master.

[nemesifier]

Thanks @pandafy, this is the proper fix for #848. The ReadOnlyJsonFieldMixin approach, swapping JSON field widgets to a ReadOnlyJsonWidget (with disabled=True) for users who lack change permission, is the right way around Django's limitation that a single widget cannot serve both editable and read-only contexts. Handling both models.JSONField and jsonfield.JSONField, covering the inline path via get_formset, and including tests all make this a clean, mergeable change.

Note: there is a competing PR for the same issue (#1327) which is full of accidentally committed artifacts (a db.sqlite3, an anti_gravity.py script that disables GIS, etc.). This PR is the correct one; #1327 should be closed in its favor.

A couple of things to confirm before merge:

1. Make sure this plays well with the advanced JSON editor: a view-only user should get the read-only rendering, and the change is scoped so it never downgrades the editable widget for users who do have change permission.
2. Confirm coverage extends to all the JSON fields users actually see read-only (device config, context, DeviceGroup context, template config), so the fix is complete rather than only on one form.
3. Docs box is unchecked; a short note is fine since this is mostly transparent.

Good work, this is close to ready.