⚡ Optimized performance of integrity check DB calls

refs #252

- the referenced issue describes performance problems with the integrity check code in knex-migrator
- this commit solves half of the problem - 1 DB call per migration minor version folder
- this results in 70+ (and growing!) DB calls at boot, with each one going back and forth to the DB... not efficient
- what the code actually needed was a count of how many migrations are currently in the DB, group by minor version - this is very easy to do in SQL and therefore knex
- this commit switches the code around to group all the DB migrations first, and then compares against the folders locally
- this results in a drop of SQL queries from 70+ to 1 🚀

lib/index.js

@@ -1132,7 +1132,6 @@ KnexMigrator.prototype._integrityCheck = function _integrityCheck(options) {
         subfolder = this.subfolder,
         force = options.force,
         folders = [],
-        operations = {},
         toReturn = {},
         futureVersions = [];
 
@@ -1148,29 +1147,69 @@ KnexMigrator.prototype._integrityCheck = function _integrityCheck(options) {
         // ignore
     }
 
-    _.each(folders, function (folder) {
-        // CASE: versions/1.1-members or versions/2.0-payments
-        if (folder !== 'init') {
-            try {
-                folder = folder.match(/([\d._]+)/)[0];
-            } catch (err) {
-                logging.warn('Cannot parse folder name.');
-                logging.warn('Ignore Folder: ' + folder);
-                return;
-            }
-        }
+    return this
+        .connection('migrations')
+        .select('version')
+        .count('version', {as: 'c'})
+        .groupBy('version')
+        .then((dbMigrations) => {
+            _.each(folders, function (folder) {
+                // CASE: versions/1.1-members or versions/2.0-payments
+                if (folder !== 'init') {
+                    try {
+                        folder = folder.match(/([\d._]+)/)[0];
+                    } catch (err) {
+                        logging.warn('Cannot parse folder name.');
+                        logging.warn('Ignore Folder: ' + folder);
+                        return;
+                    }
+                }
+
+                // CASE:
+                // if your current version is 1.0 and you add migration scripts for the next version 1.1
+                // we won't execute them until your current version changes to 1.1 or until you force KM to migrate to it
+                if (self.currentVersion && !force) {
+                    if (utils.isGreaterThanVersion({smallerVersion: self.currentVersion, greaterVersion: folder})) {
+                        futureVersions.push(folder);
+                    }
+                }
+
+                let actual = 0;
+                let expected;
+
+                const migrationCount = dbMigrations.find(m => m.version === folder);
+                if (migrationCount) {
+                    actual = migrationCount.c;
+                }
+
+                if (folder !== 'init') {
+                    expected = utils.listFiles(path.join(self.migrationPath, subfolder, folder)).length;
+                } else {
+                    expected = utils.listFiles(path.join(self.migrationPath, folder)).length;
+                }
 
-        // CASE:
-        // if your current version is 1.0 and you add migration scripts for the next version 1.1
-        // we won't execute them until your current version changes to 1.1 or until you force KM to migrate to it
-        if (self.currentVersion && !force) {
-            if (utils.isGreaterThanVersion({smallerVersion: self.currentVersion, greaterVersion: folder})) {
-                futureVersions.push(folder);
+                debug('Version ' + folder + ' expected: ' + expected);
+                debug('Version ' + folder + ' actual: ' + actual);
+
+                toReturn[folder] = {
+                    expected: expected,
+                    actual: actual
+                };
+            });
+
+            // CASE: ensure that either you have to run `migrate --force` or they ran already
+            if (futureVersions.length) {
+                _.each(futureVersions, function (futureVersion) {
+                    if (toReturn[futureVersion].actual !== toReturn[futureVersion].expected) {
+                        logging.warn('knex-migrator is skipping ' + futureVersion);
+                        logging.warn('Current version in MigratorConfig.js is smaller then requested version, use --force to proceed!');
+                        logging.warn('Please run `knex-migrator migrate --v ' + futureVersion + ' --force` to proceed!');
+                        delete toReturn[futureVersion];
+                    }
+                });
             }
-        }
 
-        operations[folder] = self.connection('migrations').where({
-            version: folder
+            return toReturn;
         }).catch(function onMigrationsLookupError(err) {
             // CASE: no database selected (database.connection.database="")
             if (err.errno === 1046) {
@@ -1199,43 +1238,6 @@ KnexMigrator.prototype._integrityCheck = function _integrityCheck(options) {
 
             throw err;
         });
-    });
-
-    return Promise.props(operations)
-        .then(function (result) {
-            _.each(result, function (value, version) {
-                let actual = value.length,
-                    expected = actual;
-
-                if (version !== 'init') {
-                    expected = utils.listFiles(path.join(self.migrationPath, subfolder, version)).length;
-                } else {
-                    expected = utils.listFiles(path.join(self.migrationPath, version)).length;
-                }
-
-                debug('Version ' + version + ' expected: ' + expected);
-                debug('Version ' + version + ' actual: ' + actual);
-
-                toReturn[version] = {
-                    expected: expected,
-                    actual: actual
-                };
-            });
-
-            // CASE: ensure that either you have to run `migrate --force` or they ran already
-            if (futureVersions.length) {
-                _.each(futureVersions, function (futureVersion) {
-                    if (toReturn[futureVersion].actual !== toReturn[futureVersion].expected) {
-                        logging.warn('knex-migrator is skipping ' + futureVersion);
-                        logging.warn('Current version in MigratorConfig.js is smaller then requested version, use --force to proceed!');
-                        logging.warn('Please run `knex-migrator migrate --v ' + futureVersion + ' --force` to proceed!');
-                        delete toReturn[futureVersion];
-                    }
-                });
-            }
-
-            return toReturn;
-        });
 };
 
 module.exports = KnexMigrator;