class DynamicPagesController < ApplicationController
  def map
    @fields = ['Software Engineering',
        'Mechanical Engineering',
        'Electrical Engineering',
        'Aerospace Engineering',
        'Civil Engineering',
        'Chemical Engineering',
        'Architectural Engineering']
  end
end

<%= select(:primary_field, :id, @fields) %>
<%= button_to "Filter", map_path, :method => :get, class: "btn btn-primary" %>

...

for (var i = 0; i < companies.length; i++){
    markers.addLayer(new L.Marker([companies[i].latitude, companies[i].longitude]).bindPopup(companies[i].popup, { maxHeight:150 }).openPopup());
}

if params[:filter].present?
    # This will apply the filter
    Model.where(attribute: params[:filter])
else
    # This will return all records
    Model.all
end

select(:primary_field, :id, @fields, allow_blank: true)