fix: add default position to dash export

[betodealmeida]

SUMMARY

The layout of dashboards is stored in the position_json attribute, containing the position and ID of charts. When a dashboard layout hasn't been modified the attribute is empty, and the relationship between which charts are contained in a given dashboard exists only in the dashboard_slices table and is not exported.

With this PR, if position_json is empty we add a default position to the attribute, so that the relationship between dashboards and charts is present in the export bundle. This doesn't change the layout, but allows us to extract the relationship when importing the dashboard together with the charts.

One alternative to this PR would be explicitly listing the UUIDs of charts when we export a dashboard, eg:

# dashboards/some_dashboard.yaml
dashboard_title: Test Import [copy]
description: null
css: ''
slug: null
uuid: a1a7dd04-d113-46b8-8cb9-29b9eef280f4
chart_uuids:
- foo
- bar

But since most dashboards will have the position_json attribute set, it seems more efficient to populate it when it's empty.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

Exported dash, verified the YAML file, and imported again, ensuring that layout was not altered.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #12007 (b7fd1d2) into master (9e07e10) will increase coverage by 0.04%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master   #12007      +/-   ##
==========================================
+ Coverage   63.59%   63.64%   +0.04%     
==========================================
  Files         939      943       +4     
  Lines       45580    45702     +122     
  Branches     4371     4388      +17     
==========================================
+ Hits        28988    29087      +99     
- Misses      16416    16438      +22     
- Partials      176      177       +1     

Flag	Coverage Δ
javascript	62.87% <ø> (+0.13%)	⬆️
python	64.10% <46.66%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/dashboards/commands/export.py	77.08% <46.66%> (-14.10%)	⬇️
superset/views/alerts.py	75.00% <0.00%> (-4.42%)	⬇️
superset/db_engines/hive.py	82.14% <0.00%> (-3.58%)	⬇️
superset/datasets/api.py	91.24% <0.00%> (-0.89%)	⬇️
superset/reports/logs/api.py	95.34% <0.00%> (ø)
superset-frontend/src/views/App.tsx	0.00% <0.00%> (ø)
superset/charts/commands/importers/v1/__init__.py	100.00% <0.00%> (ø)
...uperset/datasets/commands/importers/v1/__init__.py	100.00% <0.00%> (ø)
...perset/databases/commands/importers/v1/__init__.py	100.00% <0.00%> (ø)
...erset/dashboards/commands/importers/v1/__init__.py	100.00% <0.00%> (ø)
... and 10 more

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[hughhhh]

i'd make height and width constants at the top

[betodealmeida]

Good point, let me do that!

[hughhhh]

👍