project.rb

class Project
  has_many :project_blocks
  has_many :upcoming_blocks, ->{ within(Date.current.beginning_of_week, 3.months.from_now.end_of_week) }, class_name: "ProjectBlock"
end

project_block.rb

class ProjectBlock
  def self.within(start_date, end_date)
    timeframe = start_date..end_date
    where(start_date: timeframe, end_date: timeframe)
  end
end

projects_controller.rb

class ProjectsController < ApplicationController
  def index
    @projects = Project.include(:upcoming_blocks)
  end
end