:start_date
:end_date
:number_of_collaborators
:sector

:leader_id
:maximum_no_of_collaborators
:name

:name
:contact_number
:email
:region

belongs_to :group

belongs_to :leader
has_many :projects

has_many :groups

:available_from
:available_until
:number_of_collaborators

@projects = Project.where("start_date >= ? AND end_date >= ?",
        Date.parse(params[:available_from]).strftime("%Y-%m-%d"),
        Date.parse(params[:available_until]).strftime("%Y-%m-%d"))

<% @projects.each do |project| %>
    <% if ((project.group.capacity - project.number_of_collaborators) >0) %>
        ...code to display results...
    <% end %>
<% end %>

@available_groups = []
@leaders = []

@projects.each do |project|
    if ((project.group.capacity - params[:number_of_collaborators]) >0)
        @available_groups << project
    end
end

@available_groups.each do |grou|
    @leaders << group.leader
end

Array can't be coerced into Fixnum