with_model_spec.rb

require "rails_helper"

describe "index issue" do
  with_model :MidsizeNameChild do
    table do |t|
      t.belongs_to :midsize_name_parent,
                   index: { name: "shorter_index_name" }
    end

    model do
      belongs_to :midsize_name_parent
    end
  end

  with_model :MidsizeNameParent do
    model do
      has_many :midsize_name_children
    end
  end

  let(:instance) { MidsizeNameParent.create }

  describe "try to instantiate" do
    it "is an example" do
      instance
    end
  end
end

Demo working/broken usage of with_model on postgres backend. Dropping L7 (and the trailing comma on L6) will cause this to blow up due to generating an index with name >63 characters (PG9.6's limit).