Seeding MongoDB using Mongoose models
It seems that you are storing the data with double links (article -> category AND category -> articles).

I assume that you need to report on articles for a category.  I would simple create an index on Category like this: 

var articleSchema = mongoose.Schema({
    category: { type: mongoose.Schema.Types.ObjectId, ref: 'Category', index: true },
    title: String
});


This way you can simplify your code a ton by keeping it KISS:

function seedArticle(categoryTitle, articleTitle) {
  CategoryModel.findOne({ title: categoryTitle }).exec()
    .then(function (category) {
      return ArticleModel.create({ title: articleTitle, category: category._id });
    });
}


Other than that, I like your code.