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+# Microchip Field Fix
+
+## Problem
+
+The microchip field in the dogs table had a `UNIQUE` constraint defined directly on the column:
+
+```sql
+microchip TEXT UNIQUE
+```
+
+In SQLite, when a `UNIQUE` constraint is applied to a nullable column, **only one row can have a NULL value**. This caused the error:
+
+```
+UNIQUE constraint failed: dogs.microchip
+```
+
+When trying to add a second dog without a microchip number.
+
+---
+
+## Solution
+
+Removed the inline `UNIQUE` constraint and replaced it with a **partial unique index** that only applies to non-NULL values:
+
+```sql
+-- Column definition (no UNIQUE constraint)
+microchip TEXT
+
+-- Partial unique index (only for non-NULL values)
+CREATE UNIQUE INDEX idx_dogs_microchip 
+ON dogs(microchip) 
+WHERE microchip IS NOT NULL;
+```
+
+### Result:
+- Multiple dogs can have no microchip (NULL values allowed)
+- Dogs with microchips still cannot have duplicates
+- Field is now truly optional
+
+---
+
+## Migration Required
+
+If you have an existing database with the old schema, you **must run the migration** before the fix will work.
+
+### Option 1: Docker Container (Recommended)
+
+```bash
+# Enter the running container
+docker exec -it breedr sh
+
+# Run the migration script
+node server/db/migrate_microchip.js
+
+# Exit container
+exit
+
+# Restart the container to apply changes
+docker restart breedr
+```
+
+### Option 2: Direct Node Execution
+
+```bash
+cd /path/to/breedr
+node server/db/migrate_microchip.js
+```
+
+### Option 3: Rebuild from Scratch (Data Loss)
+
+```bash
+# Stop container
+docker stop breedr
+
+# Remove old database
+rm /mnt/user/appdata/breedr/breedr.db
+
+# Start container (will create fresh database)
+docker start breedr
+```
+
+**Warning:** Option 3 deletes all data. Only use if you have no important data or have a backup.
+
+---
+
+## What the Migration Does
+
+### Step-by-Step Process
+
+1. **Check Database Exists** - Skips if no database found
+2. **Create New Table** - With corrected schema (no UNIQUE on microchip)
+3. **Copy All Data** - Transfers all dogs from old table to new
+4. **Drop Old Table** - Removes the table with bad constraint
+5. **Rename New Table** - Makes new table the primary dogs table
+6. **Create Partial Index** - Adds unique index only for non-NULL microchips
+7. **Recreate Indexes** - Restores name and registration indexes
+8. **Recreate Triggers** - Restores updated_at timestamp trigger
+
+### Safety Features
+
+- **Idempotent** - Can be run multiple times safely
+- **Data Preservation** - All data is copied before old table is dropped
+- **Foreign Keys** - Temporarily disabled during migration
+- **Error Handling** - Clear error messages if something fails
+
+---
+
+## Verification
+
+After migration, you should be able to:
+
+### Test 1: Add Dog Without Microchip
+
+```bash
+curl -X POST http://localhost:3000/api/dogs \
+  -H "Content-Type: application/json" \
+  -d '{
+    "name": "Test Dog 1",
+    "breed": "Golden Retriever",
+    "sex": "male"
+  }'
+```
+
+**Expected:** Success (no microchip error)
+
+### Test 2: Add Another Dog Without Microchip
+
+```bash
+curl -X POST http://localhost:3000/api/dogs \
+  -H "Content-Type: application/json" \
+  -d '{
+    "name": "Test Dog 2",
+    "breed": "Labrador",
+    "sex": "female"
+  }'
+```
+
+**Expected:** Success (multiple NULL microchips allowed)
+
+### Test 3: Add Dog With Microchip
+
+```bash
+curl -X POST http://localhost:3000/api/dogs \
+  -H "Content-Type: application/json" \
+  -d '{
+    "name": "Test Dog 3",
+    "breed": "Beagle",
+    "sex": "male",
+    "microchip": "985112345678901"
+  }'
+```
+
+**Expected:** Success
+
+### Test 4: Try Duplicate Microchip
+
+```bash
+curl -X POST http://localhost:3000/api/dogs \
+  -H "Content-Type: application/json" \
+  -d '{
+    "name": "Test Dog 4",
+    "breed": "Poodle",
+    "sex": "female",
+    "microchip": "985112345678901"
+  }'
+```
+
+**Expected:** Error (duplicate microchip not allowed)
+
+---
+
+## Database Schema Comparison
+
+### Before (Broken)
+
+```sql
+CREATE TABLE dogs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  name TEXT NOT NULL,
+  breed TEXT NOT NULL,
+  sex TEXT NOT NULL,
+  microchip TEXT UNIQUE,  -- ❌ Only one NULL allowed
+  ...
+);
+```
+
+### After (Fixed)
+
+```sql
+CREATE TABLE dogs (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  name TEXT NOT NULL,
+  breed TEXT NOT NULL,
+  sex TEXT NOT NULL,
+  microchip TEXT,  -- ✓ Multiple NULLs allowed
+  ...
+);
+
+-- Partial unique index
+CREATE UNIQUE INDEX idx_dogs_microchip 
+ON dogs(microchip) 
+WHERE microchip IS NOT NULL;  -- ✓ Only enforces uniqueness on non-NULL
+```
+
+---
+
+## Technical Details
+
+### Why SQLite Behaves This Way
+
+From SQLite documentation:
+
+> For the purposes of UNIQUE constraints, NULL values are considered distinct from all other values, including other NULLs. However, when a UNIQUE constraint is defined on a column, SQLite treats NULL as a single value.
+
+This is a quirk of SQLite's implementation. PostgreSQL and MySQL allow multiple NULLs in UNIQUE columns by default.
+
+### Partial Index Solution
+
+Partial indexes (with WHERE clause) were introduced in SQLite 3.8.0 (2013). They allow us to:
+
+1. Create an index that only includes certain rows
+2. In this case, only rows where `microchip IS NOT NULL`
+3. This means the uniqueness constraint doesn't apply to NULL values
+4. Multiple NULL values are now allowed
+
+---
+
+## Rollback (If Needed)
+
+If something goes wrong during migration:
+
+### Manual Rollback Steps
+
+1. **Stop the application**
+   ```bash
+   docker stop breedr
+   ```
+
+2. **Restore from backup** (if you made one)
+   ```bash
+   cp /mnt/user/appdata/breedr/breedr.db.backup \
+      /mnt/user/appdata/breedr/breedr.db
+   ```
+
+3. **Start the application**
+   ```bash
+   docker start breedr
+   ```
+
+### Create Backup Before Migration
+
+```bash
+# Stop container
+docker stop breedr
+
+# Create backup
+cp /mnt/user/appdata/breedr/breedr.db \
+   /mnt/user/appdata/breedr/breedr.db.backup
+
+# Start container
+docker start breedr
+
+# Run migration
+docker exec -it breedr node server/db/migrate_microchip.js
+```
+
+---
+
+## Future Prevention
+
+All new databases created with the updated schema will have the correct constraint from the start. No migration needed for:
+
+- Fresh installations
+- Deleted and recreated databases
+- Databases created after this fix
+
+---
+
+## Related Files
+
+- **Schema Definition:** `server/db/init.js`
+- **Migration Script:** `server/db/migrate_microchip.js`
+- **This Document:** `docs/MICROCHIP_FIX.md`
+
+---
+
+## Changelog
+
+### March 8, 2026
+- Identified UNIQUE constraint issue with microchip field
+- Created migration script to fix existing databases
+- Updated schema for new databases
+- Added partial unique index solution
+- Documented problem and solution
+
+---
+
+*If you encounter any issues with the migration, check the container logs:*
+
+```bash
+docker logs breedr
+```
+
+*Or open a GitHub issue with the error details.*
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