Add migration to merge duplicate Locations

- having duplicate Location nodes in the production database blocks us
  from adding a unique constraint, so that Locations are not created
which have the same id.

backend/src/db/migrate/store.js

@@ -35,7 +35,7 @@ class Store {
     const session = driver.session()
     const readTxResultPromise = session.readTransaction(async txc => {
       const result = await txc.run(
-        'MATCH (migration:Migration) RETURN migration {.*} ORDER BY migration.timestamp DESC',
+        'MATCH (migration:Migration) RETURN migration {.*} ORDER BY migration.createdAt DESC',
       )
       return result.records.map(r => r.get('migration'))
     })
@@ -64,12 +64,19 @@ class Store {
     const { migrations } = set
     const writeTxResultPromise = session.writeTransaction(txc => {
       return Promise.all(
-        migrations.map(migration => {
-          const { title, description, timestamp } = migration
-          const properties = { title, description, timestamp }
-          return txc.run('CREATE (migration:Migration) SET migration += $properties', {
-            properties,
-          })
+        migrations.map(async migration => {
+          const { title, description } = migration
+          const properties = { title, description }
+          const migrationResult = await txc.run(
+            `
+              MERGE (migration:Migration { title: $properties.title })
+              ON CREATE SET migration += $properties,
+              migration.createdAt = toString(datetime())
+              RETURN migration
+            `,
+            { properties },
+          )
+          return migrationResult
         }),
       )
     })

backend/src/db/migrations/1579387929111-merge_duplicate_location_nodes.js

@@ -0,0 +1,77 @@
+import { throwError, concat } from 'rxjs'
+import { flatMap, mergeMap, map, catchError } from 'rxjs/operators'
+import { getDriver } from '../neo4j'
+
+export const description = `
+  This migration merges duplicate :Location nodes. It became
+  necessary after we realized that we had not set up constraints for Location.id in production.
+`
+export function up(next) {
+  const driver = getDriver()
+  const rxSession = driver.rxSession()
+  rxSession
+    .beginTransaction()
+    .pipe(
+      flatMap(transaction =>
+        concat(
+          transaction
+            .run(
+              `
+                    MATCH (location:Location)
+                    RETURN location {.id}
+                  `,
+            )
+            .records()
+            .pipe(
+              map(record => {
+                const { id: locationIds } = record.get('location')
+                return { locationIds }
+              }),
+              mergeMap(({ locationIds }) => {
+                return transaction
+                  .run(
+                    ` 
+                    MATCH(location:Location {id: $locationIds}), (location2:Location {id: $locationIds})
+                    WHERE location.id = location2.id AND id(location) < id(location2)
+                    CALL apoc.refactor.mergeNodes([location, location2], { properties: 'combine', mergeRels: true }) YIELD node as updatedLocation
+                    RETURN location {.*},updatedLocation {.*}
+                  `,
+                    { locationIds },
+                  )
+                  .records()
+                  .pipe(
+                    map(record => ({
+                      location: record.get('location'),
+                      updatedLocation: record.get('updatedLocation'),
+                    })),
+                  )
+              }),
+            ),
+          transaction.commit(),
+        ).pipe(catchError(error => transaction.rollback().pipe(throwError(error)))),
+      ),
+    )
+    .subscribe({
+      next: ({ updatedLocation, location }) =>
+        // eslint-disable-next-line no-console
+        console.log(`
+          Merged:
+          =============================
+          locationId: ${location.id}
+          updatedLocation: ${location.id} => ${updatedLocation.id}
+          =============================
+        `),
+      complete: () => {
+        // eslint-disable-next-line no-console
+        console.log('Merging of duplicate locations completed')
+        next()
+      },
+      error: error => {
+        next(new Error(error), null)
+      },
+    })
+}
+
+export function down(next) {
+  next(new Error('Irreversible migration'))
+}