refator(operation-base): support read preferences on operations

This introduces an `OperationBaseV2` which accepts a "parent" as
the first parameter of the constructor. This parent is used to
resolve the read preference, defaulting to primary.

Author: mbroadst

lib/operations/distinct.js

@@ -2,7 +2,7 @@
 
 const Aspect = require('./operation').Aspect;
 const defineAspects = require('./operation').defineAspects;
-const OperationBase = require('./operation').OperationBase;
+const OperationBaseV2 = require('./operation').OperationBaseV2;
 const decorateWithCollation = require('../utils').decorateWithCollation;
 const decorateWithReadConcern = require('../utils').decorateWithReadConcern;
 const executeCommand = require('./db_ops').executeCommand;
@@ -18,7 +18,7 @@ const resolveReadPreference = require('../utils').resolveReadPreference;
  * @property {object} query The query for filtering the set of documents to which we apply the distinct filter.
  * @property {object} [options] Optional settings. See Collection.prototype.distinct for a list of options.
  */
-class DistinctOperation extends OperationBase {
+class DistinctOperation extends OperationBaseV2 {
   /**
    * Construct a Distinct operation.
    *
@@ -28,7 +28,7 @@ class DistinctOperation extends OperationBase {
    * @param {object} [options] Optional settings. See Collection.prototype.distinct for a list of options.
    */
   constructor(collection, key, query, options) {
-    super(options);
+    super(collection, options);
 
     this.collection = collection;
     this.key = key;

lib/operations/operation.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const ReadPreference = require('../core').ReadPreference;
+
 const Aspect = {
   READ_OPERATION: Symbol('READ_OPERATION'),
   SKIP_SESSION: Symbol('SKIP_SESSION'),
@@ -44,6 +46,14 @@ class OperationBase {
   }
 }
 
+class OperationBaseV2 extends OperationBase {
+  constructor(parent, options) {
+    super(options);
+
+    this.readPreference = parent ? parent.readPreference : ReadPreference.primary;
+  }
+}
+
 function defineAspects(operation, aspects) {
   if (!Array.isArray(aspects) && !(aspects instanceof Set)) {
     aspects = [aspects];
@@ -59,5 +69,6 @@ function defineAspects(operation, aspects) {
 module.exports = {
   Aspect,
   defineAspects,
-  OperationBase
+  OperationBase,
+  OperationBaseV2
 };