Added migration for existing docs

server/migrations/20180324214403-serializer-upgrade.js

@@ -0,0 +1,26 @@
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    // upgrade to slate-md-serializer 3.0 means that newlines saved in Markdown are now
+    // accurately reflected in the editor. To prevent a change in appearance in current docs
+    // we need to collapse existing multiple newlines in the db.
+    const [documents, metaData] = await queryInterface.sequelize.query(`SELECT * FROM documents`);
+    for (const document of documents) {
+      const id = document.id;
+      const fixedText = document.text.replace(/\n{2,}/gi, "\n\n");
+      if (fixedText === document.text) continue;
+
+      // raw query to avoid hooks
+      await queryInterface.sequelize.query(`
+        update documents
+        set "text" = :fixedText
+        where id = :id
+      `, { replacements: { fixedText, id } })
+    }
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    // cannot be reversed
+  }
+};
+
+