Add unique constraint on loan_events to prevent duplicate indexing

[ogazboiz]

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Description

The event indexer can insert duplicate rows into loan_events if it re-processes the same ledger range (e.g., on restart). There is no unique constraint to prevent this.

What Needs to Happen

• Add a unique constraint on (loan_id, event_type, ledger) in loan_events
• Use INSERT ... ON CONFLICT DO NOTHING in the indexer insert query
• Add a migration for the constraint

Files

• backend/migrations/
• backend/src/services/eventIndexer.ts

Acceptance Criteria

• Duplicate events cannot be inserted
• Indexer handles conflicts gracefully on restart
• Tests verify idempotent indexing

[Daniel202251]

@Daniel202251 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I’m a smart contract and full-stack developer who turns complex ideas into secure, production-ready blockchain code.
With strong skills in Rust and Solidity, I build systems that are fast, safe, and built to last.
My experience with the Stellar SDK lets me ship real-world Web3 solutions that actually move value

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[intelliDean]

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I'd like to work on this issue

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[devfoma]

@devfoma has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

GM, Chief. I would like to take on this issue

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[drips-wave]

Congratulations, @devfoma! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @devfoma: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @devfoma as part of the Stellar Wave Program's 4th Wave 🥳

Moderator's reason: [Auto-assessed] The PR was merged and successfully addresses all requirements of the issue, including adding a unique constraint on (loan_id, event_type, ledger), updating the indexer to use 'ON CONFLICT DO NOTHING', and providing a migration that handles existing duplicates. Although the PR also refactored 'loan_events' into 'contract_events', it maintained backward compatibility and updated the relevant logic as requested.

😎 @devfoma: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.