mongodb/jstests/noPassthrough/count_helper_read_preference.js

// Tests that the read preference set on the connection is used when we call the count helper.
(function() {
"use strict";

var commandsRan = [];

// Create a new DB object backed by a mock connection.
function MockMongo() {
    this.getMinWireVersion = function getMinWireVersion() {
        return 0;
    };

    this.getMaxWireVersion = function getMaxWireVersion() {
        return 0;
    };
}
MockMongo.prototype = Mongo.prototype;
MockMongo.prototype.runCommand = function(db, cmd, opts) {
    commandsRan.push({db: db, cmd: cmd, opts: opts});
    return {ok: 1, n: 100};
};

const mockMongo = new MockMongo();
var db = new DB(mockMongo, "test");

// Attach a dummy implicit session because the mock connection cannot create sessions.
db._session = new _DummyDriverSession(mockMongo);

assert.eq(commandsRan.length, 0);

// Run a count with no readPref.
db.getMongo().setReadPref(null);
db.foo.count();

// Check that there is no readPref on the command document.
assert.eq(commandsRan.length, 1);
assert.docEq(commandsRan[0].cmd, {count: "foo", query: {}});

commandsRan = [];

// Run with readPref secondary.
db.getMongo().setReadPref("secondary");
db.foo.count();

// Check that we have wrapped the command and attached the read preference.
assert.eq(commandsRan.length, 1);
assert.docEq(commandsRan[0].cmd,
             {query: {count: "foo", query: {}}, $readPreference: {mode: "secondary"}});
})();