Ember.js: How to get an array of model IDs from a corresponding array of model attributes

For a Tag model that I have in Ember-Data, I have 4 records in my store:

Tags:

id   tag_name
1    Writing
2    Reading-Comprehension
3    Biology
4    Chemistry

In my code I have an array of tag_names, and I want to get a corresponding array of tag IDs. I'm having 2 problems:

1. My server is being queried even though I have these tags in my store. When I call store.find('tag', {tag_name: tag_name}), I didn't expect to need a call to the server. Here is all the code I'm using to attempt to create an array of IDs.

   var self = this;
   var tagsArray = ["Writing", "Reading-Comprehension", "Chemistry"];
   var tagIdArr = []
   tagsArray.forEach(function(tag_name) {
       return self.store.find('tag', { tag_name: tag_name }).then(function(tag) {
           tagIdArr.pushObject(tag.get('content').get('0').get('id'));
       })
   })
   return tagIdArr;
   
   

2. When I console.log the output of the above code gives me an empty array object with length 0. Clicking on the caret next to the empty array shows three key-value pairs with the correct data. But the array is empty. I'm sure there is a simple explanation for this behavior, but I'm not sure why this is. I've used code similar to the above in other places successfully.