refactor: adapt many sql queries for postgresql

backend/lib/helpers.js

@@ -1,4 +1,6 @@
-const moment = require('moment');
+const moment       = require('moment');
+const {isPostgres} = require('./config');
+const {ref}        = require('objection');
 
 module.exports = {
 
@@ -45,6 +47,20 @@ module.exports = {
 			}
 		});
 		return obj;
-	}
+	},
 
+	/**
+	 * Casts a column to json if using postgres
+	 *
+	 * @param {string} colName
+	 * @returns {string|Objection.ReferenceBuilder}
+	 */
+	castJsonIfNeed: function (colName) {
+		if (isPostgres()) {
+			return ref(colName).castText();
+		}
+		else {
+			return colName;
+		}
+	}
 };