[SalesForce] Not able to select multiple picklist values using lightning:select

I go pragmatic here, with an extra level of indirection to try to cope with the shifting sands of Lightning.

If I'm not sure, then I write a custom component which delegates to the lightning: version.

So, for example, I have a custom date input component like this:

<aura:component implements="flexipage:availableForAllPageTypes,c:FormQuestion" access="global" extends="c:FormElement" >
    <div aura:id="container">
        <lightning:input aura:id="inputField" type="date" value="{!v.value}" label="{!v.question}" required="{!v.required}" messageWhenBadInput="{!v.errorMessage}" messageWhenValueMissing="{!v.question + ' is required'}" validity="{!v.validity}" />
    </div>
</aura:component>

When Winter 18 is out, I can just swap in the <lightning:formattedDateTime> component instead of <lightning:input> and add any custom logic along the way to handle any oddities required during the delegation.

It does mean more components, but I think the extra layer of abstraction is worth it for now. Once the components are all out of Beta, then we can breath a sigh of relief!

The basic method today is to get the values from the server, then render the values with $A.createComponents.

doInit: function(component, event, helper) {
  var action = component.get("c.init");
  action.setParams({ ... });
  action.setCallback(this, function(result) {
    var options = [];
    result.getReturnValue().values.forEach(
      function(value) {
        options.push(['option', { label: value, value: value }]);
      });
    $A.createComponents(
      options,
      function(components) {
        component.find("mypicklist").set("v.options", components);
      });
    });
  $A.enqueueAction(action);
}

If that's too complicated, then you might consider using lightning:inputField, which is designed to work with lightning:recordEditForm.

There's no way to get lightning:select automatically populate with the options when bound to a field.