feat(read-preference): add transaction to inheritance rules

lib/utils.js

@@ -511,10 +511,15 @@ function resolveReadPreference(options, sources) {
   const db = sources.db;
   const coll = sources.collection;
   const defaultReadPreference = sources.default;
+  const session = options.session;
 
   let readPreference;
   if (options.readPreference) {
     readPreference = options.readPreference;
+  } else if (session && session.inTransaction() && session.transaction.options.readPreference) {
+    // From transactions spec: If the user supplies an explicit readConcern via a method
+    // option, however, drivers MUST apply the readConcern...
+    readPreference = session.transaction.options.readPreference;
   } else if (coll && coll.s.readPreference) {
     readPreference = coll.s.readPreference;
   } else if (db && db.s.readPreference) {
@@ -583,20 +588,18 @@ function decorateWithCollation(command, target, options) {
   }
 }
 
-function decorateWithReadConcern(command, coll, options) {
+/**
+ * Applies a read concern to a given command.
+ *
+ * @param {object} command the command on which to apply the read concern
+ * @param {Collection} [target] the parent collection of the operation calling this method
+ */
+function decorateWithReadConcern(command, coll) {
   let readConcern = Object.assign({}, command.readConcern || {});
   if (coll.s.readConcern) {
     Object.assign(readConcern, coll.s.readConcern);
   }
 
-  if (
-    options.session &&
-    options.session.supports.causalConsistency &&
-    options.session.operationTime
-  ) {
-    Object.assign(readConcern, { afterClusterTime: options.session.operationTime });
-  }
-
   if (Object.keys(readConcern).length > 0) {
     Object.assign(command, { readConcern: readConcern });
   }