Added segmeted email batch creation

refs https://github.com/TryGhost/Team/issues/828

- When sending email batches out they need to be created without mixing different member segments. This allows for easier reasoning about what data has been sent out to each specific email recipient
- Modified email batches to chunk based on segments defined in the HTML content of the post

core/server/services/mega/mega.js

@@ -16,6 +16,7 @@ const jobsService = require('../jobs');
 const db = require('../../data/db');
 const models = require('../../models');
 const postEmailSerializer = require('./post-email-serializer');
+const {getSegmentsFromHtml} = require('./segment-parser');
 
 const getFromAddress = () => {
     let fromAddress = membersService.config.getEmailFromAddress();
@@ -277,7 +278,7 @@ async function sendEmailJob({emailModel, options}) {
             let newBatchCount;
 
             await models.Base.transaction(async (transacting) => {
-                newBatchCount = await createEmailBatches({emailModel, options: {transacting}});
+                newBatchCount = await createSegmentedEmailBatches({emailModel, options: {transacting}});
             });
 
             if (newBatchCount === 0) {
@@ -311,10 +312,19 @@ async function sendEmailJob({emailModel, options}) {
     }
 }
 
-// Fetch rows of members that should receive an email.
-// Uses knex directly rather than bookshelf to avoid thousands of bookshelf model
-// instantiations and associated processing and event loop blocking
-async function getEmailMemberRows({emailModel, options}) {
+/**
+ * Fetch rows of members that should receive an email.
+ * Uses knex directly rather than bookshelf to avoid thousands of bookshelf model
+ * instantiations and associated processing and event loop blocking
+ *
+ * @param {Object} options
+ * @param {Object} options.emailModel - instance of Email model
+ * @param {string} [options.memberSegment] - NQL filter to apply in addition to the one defined in emailModel
+ * @param {Object} options.options - knex options
+ *
+ * @returns {Promise<Object[]>} instances of filtered knex member rows
+ */
+async function getEmailMemberRows({emailModel, memberSegment, options}) {
     const knexOptions = _.pick(options, ['transacting', 'forUpdate']);
     const filterOptions = Object.assign({}, knexOptions);
 
@@ -334,6 +344,10 @@ async function getEmailMemberRows({emailModel, options}) {
         filterOptions.filter = `subscribed:true+${recipientFilter}`;
     }
 
+    if (memberSegment) {
+        filterOptions.filter = `${filterOptions.filter}+${memberSegment}`;
+    }
+
     const startRetrieve = Date.now();
     debug('getEmailMemberRows: retrieving members list');
     // select('members.*') is necessary here to avoid duplicate `email` columns in the result set
@@ -344,6 +358,34 @@ async function getEmailMemberRows({emailModel, options}) {
     return memberRows;
 }
 
+/**
+ * Detects segment filters in emailModel's html and creates separate batches per segment
+ *
+ * @param {Object} options
+ * @param {Object} options.emailModel - instance of Email model
+ * @param {Object} options.options - knex options
+ */
+async function createSegmentedEmailBatches({emailModel, options}) {
+    const segments = getSegmentsFromHtml(emailModel.get('html'));
+    const batchIds = [];
+
+    if (segments.length) {
+        for (const memberSegment of segments) {
+            const emailBatchIds = await createEmailBatches({
+                emailModel,
+                memberSegment,
+                options
+            });
+            batchIds.push(emailBatchIds);
+        }
+    } else {
+        const emailBatchIds = createEmailBatches({emailModel, options});
+        batchIds.push(emailBatchIds);
+    }
+
+    return batchIds;
+}
+
 /**
  * Store email_batch and email_recipient records for an email.
  * Uses knex directly rather than bookshelf to avoid thousands of bookshelf model
@@ -351,12 +393,12 @@ async function getEmailMemberRows({emailModel, options}) {
  *
  * @param {Object} options
  * @param {Object} options.emailModel - instance of Email model
- * @param {Object} options.options
- *
+ * @param {string} [options.memberSegment] - NQL filter to apply in addition to the one defined in emailModel
+ * @param {Object} options.options - knex options
  * @returns {Promise<string[]>} - created batch ids
  */
-async function createEmailBatches({emailModel, options}) {
-    const memberRows = await getEmailMemberRows({emailModel, options});
+async function createEmailBatches({emailModel, memberSegment, options}) {
+    const memberRows = await getEmailMemberRows({emailModel, memberSegment, options});
 
     if (!memberRows.length) {
         return [];
@@ -364,6 +406,7 @@ async function createEmailBatches({emailModel, options}) {
 
     const storeRecipientBatch = async function (recipients) {
         const knexOptions = _.pick(options, ['transacting', 'forUpdate']);
+        // TODO: store `memberSegment` in EmailBatch once the table migration is merged
         const batchModel = await models.EmailBatch.add({email_id: emailModel.id}, knexOptions);
 
         const recipientData = [];