Has many :through association

Synopsys:

class Grid < ActiveRecord::Base
    
  has_many :block_grid_associations, :dependent => :delete_all, :autosave => true, :include => :block
  has_many :blocks, :through => :block_grid_associations
  
  # if you need ordered blocks inside each grid (assuming a position column in block_grid_associations table)
  def block_ids=(ids)
    unless (ids = ids.map(&:to_i).select{|i|i>0}) == (current_ids = block_grid_associations.map(&:block_id))
      (current_ids - ids).each { |id| block_grid_associations.select{|b|b.block_id == id}.first.mark_for_destruction }
      self.blocks = ids.map_with_index do |id, index|
        if current_ids.include?(id)
          (block_association = block_grid_associations.select{|b|b.block_id == id}.first).position = (index+1)
          block_association
        else
          block_grid_associations.build({:block_id => id, :position => (index+1)})
        end
      end.map(&:block)
    end
  end

  # for a multiselect widget: (natural choice for n-n associations)

    attr_accessible :block_ids

  # for a nested form: 
   
    accepts_nested_attributes_for :blocks, :allow_destroy => true
    attr_accessible :blocks_attributes

end

# for info

class Block < ActiveRecord::Base
  has_many :block_grid_associations, :dependent => :delete_all
  has_many :grids, :through => :block_grid_associations
end

class BlockGridAssociation < ActiveRecord::Base
  belongs_to :block
  belongs_to :grid
  default_scope order(:position) # if you need ordered blocks
end

RailsAdmin.config |config| do
  config.model Grid do
    configure :block_grid_associations do
      visible(false)
    end

    configure :blocks do
      # configuration here
    end
  end
end

Note: has_many :through associations are not considered any differently from vanilla has_many association. In particular, no special help is provided to edit join table attributes; you can edit indifferently the join-table or the target table.

More here (has_many)