fix: add unique index on subscriptions to prevent duplicate entries#2553
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mroderick merged 2 commits intocodebar:masterfrom Apr 9, 2026
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Members subscribed to multiple groups within a chapter were being processed multiple times when sending invitations with audience='everyone', causing 'Member has already been taken' validation errors. Root Cause: - Member.in_group() uses JOIN on subscriptions, returning duplicate rows when a member is subscribed to the same group type multiple times - 336 members in London chapter affected (subscribed to both Students and Coaches) - 35 members in Berlin chapter affected - 57 members in Brighton chapter affected Impact: - Workshop invitation batches failed mid-process (e.g., 517 of 7000+ invitations sent before failure) - InvitationLogEntry validates uniqueness of member_id per invitation, causing validation errors on second processing attempt - Admins had to manually retry batch sends multiple times Affected Chapters: - 46 of 54 chapters (85%) have members with duplicate subscriptions - ~1,200 members affected across all chapters - Top 5: London (336), West London (79), Brighton (57), South London (57), Barcelona (55), Edinburgh (37), Berlin (35) Timeline: - Original code (Jan 2017, commit 448acc7) included .uniq for deduplication - Refactoring (May 2018, commit d7a463d) extracted to scopes but accidentally removed the .uniq call Fix: Add .distinct to chapter_students and chapter_coaches methods, restoring the deduplication behavior that existed before the refactoring. Tests: - Add spec for unique members when subscribed to same group multiple times - Add spec for unique invitations per role when member is in both groups
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- Uses disable_ddl_transaction! (https://api.rubyonrails.org/classes/ActiveRecord/Migration.html#method-i-disable_ddl_transaction-21) - Uses algorithm: :concurrently (https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index) - Removes 31 duplicate subscriptions from production database - Prevents race conditions from causing duplicate entries in future Why (member_id, group_id) not (member_id, chapter_id)? - Index on (member_id, group_id) allows a member to be both student AND coach in the same chapter - Each chapter has separate "Students" and "Coaches" groups - different group_id values - This index prevents duplicate subscriptions to the SAME group (the race condition bug) - But allows valid dual membership: member can subscribe to group_id=1 (Students) AND group_id=2 (Coaches)
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Please review #2551 first, and then this one ... or roll a 1d20 sanity check 🎲
Summary
Adds a database-level unique index on
(member_id, group_id)in the subscriptions table to prevent race conditions that cause duplicate subscriptions.Problem
The Rails model validation
validates :group, uniqueness: { scope: :member_id }can be bypassed by race conditions when two concurrent requests both pass validation before either saves to the database.Evidence from Production
Analysis of production data revealed 25 unique member/group combinations with duplicate subscriptions created within 3ms to 2.5 seconds of each other - classic race condition pattern spanning 2015-2023.
Solution
(member_id, group_id)to enforce at database levelWhy (member_id, group_id) not (member_id, chapter_id)?
The index on
(member_id, group_id)allows a member to be both a student AND a coach in the same chapter:group_idgroup_id=1(London Students) ANDgroup_id=2(London Coaches)Using
(member_id, chapter_id)would incorrectly prevent the student+coach combination.Migration Details
disable_ddl_transaction!to allow concurrent index creationalgorithm: :concurrentlyto avoid table locks during production deploymentsTesting
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