> Mysql2::Error: Column 'priority_id' cannot be null:

As log says, Redmine core does set priority null.
Your plugin causes it.
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I was able to reproduce without plugins: Create issue, create subtask and assign it, then change status and on the close it fails.
<pre>
# ruby ./bin/about

Environment:
Redmine version 3.3.0.stable.15732
Ruby version 1.9.3-p327 (2012-11-10) [i686-linux]
Rails version 4.2.6
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.8
Git 1.7.1
Filesystem
Redmine plugins:
no plugin installed

# tail -n 50 log/production.log

Started GET "/issues/18256" for 127.0.0.1 at 2016-09-21 14:23:47 +0300
Processing by IssuesController#show as HTML
Parameters: {"id"=>"18256"}
Current user: ppo (id=95)
Rendered issues/_action_menu.html.erb (4.3ms)
Rendered issue_relations/_form.html.erb (2.3ms)
Rendered issues/_relations.html.erb (3.4ms)
Rendered issues/_action_menu.html.erb (3.5ms)
Rendered issues/_form_custom_fields.html.erb (1.5ms)
Rendered issues/_attributes.html.erb (25.2ms)
Rendered issues/_form.html.erb (96.4ms)
Rendered attachments/_form.html.erb (2.7ms)
Rendered issues/_edit.html.erb (366.8ms)
Rendered issues/_sidebar.html.erb (7.5ms)
Rendered watchers/_watchers.html.erb (3.4ms)
Rendered issues/show.html.erb within layouts/base (559.1ms)
Completed 200 OK in 677ms (Views: 416.2ms | ActiveRecord: 190.3ms)
Started PATCH "/issues/18256/edit.js" for 127.0.0.1 at 2016-09-21 14:26:35 +0300
Processing by IssuesController#edit as JS
Parameters: {"utf8"=>"✓", "authenticity_token"=>"zOohZFn5pbDzVhvKxCAOvVgK3GYxuPxK9r23Xl53ThGc/HT1/eJbc6k64eyHRIxBLVD6A7mE1IiKOih7JY+a8A==", "form_update_triggered_by"=>"issue_status_id", "issue"=>{"is_private"=>"0", "project_id"=>"78", "tracker_id"=>"4", "subject"=>"Child issue", "description"=>"", "status_id"=>"5", "priority_id"=>"3", "assigned_to_id"=>"95", "category_id"=>"", "parent_issue_id"=>"18255", "start_date"=>"2016-09-21", "due_date"=>"", "estimated_hours"=>"", "done_ratio"=>"0", "custom_field_values"=>{"23"=>"", "30"=>""}, "notes"=>"", "private_notes"=>"0", "lock_version"=>"0"}, "time_entry"=>{"hours"=>"", "activity_id"=>"20", "user_id"=>"95", "comments"=>""}, "last_journal_id"=>"", "next_issue_id"=>"18255", "issue_position"=>"1", "issue_count"=>"11", "id"=>"18256"}
Current user: ppo (id=95)
Rendered issues/_form_custom_fields.html.erb (3.1ms)
Rendered issues/_attributes.html.erb (26.8ms)
Rendered issues/_form.html.erb (103.1ms)
Rendered issues/edit.js.erb (251.5ms)
Completed 200 OK in 342ms (Views: 248.2ms | ActiveRecord: 16.8ms)
Started PATCH "/issues/18256" for 127.0.0.1 at 2016-09-21 14:26:37 +0300
Processing by IssuesController#update as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"zOohZFn5pbDzVhvKxCAOvVgK3GYxuPxK9r23Xl53ThGc/HT1/eJbc6k64eyHRIxBLVD6A7mE1IiKOih7JY+a8A==", "form_update_triggered_by"=>"", "issue"=>{"is_private"=>"0", "project_id"=>"78", "tracker_id"=>"4", "subject"=>"Child issue", "description"=>"", "status_id"=>"5", "priority_id"=>"3", "assigned_to_id"=>"95", "category_id"=>"", "parent_issue_id"=>"18255", "start_date"=>"2016-09-21", "due_date"=>"", "estimated_hours"=>"", "done_ratio"=>"0", "custom_field_values"=>{"23"=>"", "30"=>""}, "notes"=>"", "private_notes"=>"0", "lock_version"=>"0"}, "time_entry"=>{"hours"=>"", "activity_id"=>"20", "user_id"=>"95", "comments"=>""}, "last_journal_id"=>"", "commit"=>"Submit", "next_issue_id"=>"18255", "issue_position"=>"1", "issue_count"=>"11", "id"=>"18256"}
Current user: ppo (id=95)
Completed 500 Internal Server Error in 175ms (ActiveRecord: 60.3ms)

ActiveRecord::StatementInvalid (Mysql2::Error: Column 'priority_id' cannot be null: UPDATE `issues` SET `issues`.`priority_id` = NULL, `issues`.`done_ratio` = 100, `issues`.`updated_on` = '2016-09-21 14:26:37', `issues`.`lock_version` = 1 WHERE `issues`.`id` = 18255 AND `issues`.`lock_version` = 0):
app/models/issue.rb:210:in `create_or_update'
app/models/issue.rb:1566:in `recalculate_attributes_for'
app/models/issue.rb:1522:in `update_parent_attributes'
app/models/issue.rb:210:in `create_or_update'
app/controllers/issues_controller.rb:512:in `block in save_issue_with_child_records'
app/controllers/issues_controller.rb:500:in `save_issue_with_child_records'
app/controllers/issues_controller.rb:179:in `update'
lib/redmine/sudo_mode.rb:63:in `sudo_mode'
</pre>
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Toshi MARUYAMA wrote:
> > Mysql2::Error: Column 'priority_id' cannot be null:
>
> As log says, Redmine core does set priority null.

Sorry, I missed *NOT*.
Redmine core does *NOT* set priority null.
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If redmine doesn't set NULL for priority and there are no plugins installed and there are no local modifications how can I track down where is problem comes from?
This is only occurs for subtasks.
<pre>
# svn info
Path: .
Working Copy Root Path: /var/www/rm330
URL: https://svn.redmine.org/redmine/branches/3.3-stable
Relative URL: ^/branches/3.3-stable
Repository Root: https://svn.redmine.org/redmine
Repository UUID: e93f8b46-1217-0410-a6f0-8f06a7374b81
Revision: 15732
Node Kind: directory
Schedule: normal
Last Changed Author: marutosijp
Last Changed Rev: 15722
Last Changed Date: 2016-07-20 08:35:54 +0300 (Wed, 20 Jul 2016)

# svn diff
#
</pre>
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source:tags/3.3.0/app/models/issue.rb#L1529

As I changed subject, issue priority default is not defined.
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Fix committed in r15857, thanks for pointing this out.
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