Minimal reproducible example: Parent node labels mssing

mre.dot

strict digraph structs {
	graph [bb="0,0,582,464"];
	node [label="\N"];
	subgraph cluster_h1a_h1c {
		graph [bb="8,8,252,456",
			label="Host (h1d):h1a",
			lheight=0.21,
			lp="130,444.5",
			lwidth=1.14
		];
		subgraph cluster_h1a_h1c_1 {
			graph [bb="16,16,238,343",
				label="Type: 4",
				lheight=0.21,
				lp="127,331.5",
				lwidth=0.60
			];
			h1a_1_a			 [height=0.5,
				label=h1a_1_a,
				pos="108,294",
				width=1.1152];
			h1a_1_b			 [height=0.5,
				label=h1a_1_b,
				pos="180,165",
				width=1.1243];
			h1a_1_a -> h1a_1_b			 [label="aa to\naa \n(Relation 900)",
				lp="89.5,229",
				pos="e,152.02,178.1 80.531,280.85 68.886,274.15 56.52,264.59 50,252 40.6,233.84 37.342,222.05 50,206 71.223,179.08 92.334,198.39 125,\
188 130.71,186.18 136.68,184.05 142.48,181.84"];
			h1a_1_b -> h1a_1_a			 [label="at to\nat \n(Relation 901)",
				lp="190.5,229",
				pos="e,116.69,276.3 167.17,182.28 161.86,189.37 155.82,197.89 151,206 139.41,225.48 139.06,231.69 129,252 126.58,256.89 123.96,262.09 \
121.41,267.08"];
			h1a_1_c			 [height=0.5,
				label=h1a_1_c,
				pos="107,42",
				width=1.1152];
			h1a_1_b -> h1a_1_c			 [label="at to\nat \n(Relation 902)",
				lp="190.5,101",
				pos="e,117.92,59.5 167.17,147.72 161.86,140.63 155.82,132.11 151,124 139.41,104.52 139.94,97.848 129,78 127.2,74.727 125.22,71.338 123.21,\
68.015"];
			h1a_1_c -> h1a_1_a			 [label="aa to\naa \n(Relation 905)",
				lp="73.5,165",
				pos="e,76.67,282.72 72.034,50.981 57.676,56.165 42.461,64.559 34,78 23.108,95.301 33.681,103.56 34,124 34.889,180.91 8.928,203.07 38,\
252 44.755,263.37 55.972,271.91 67.469,278.18"];
			h1a_1_c -> h1a_1_b			 [label="aa to\naa \n(Relation 906)",
				lp="85.5,101",
				pos="e,152.04,151.84 75.395,53.432 64.059,58.875 52.486,66.8 46,78 35.754,95.692 33.25,108.02 46,124 68.458,152.15 90.622,131.28 125,\
142 130.72,143.78 136.69,145.9 142.5,148.1"];
		}
		h1a_h1c_1		 [height=0.74639,
			label="1\n[Engine: h1c]",
			pos="180,398",
			width=1.7826];
		h1a_h1c_1 -> h1a_1_a		 [pos="e,119.59,311.42 162.39,372.05 151.15,356.12 136.64,335.57 125.43,319.69"];
		h1a_h1c_1 -> h1a_1_b		 [pos="e,205.29,179.15 191.27,371.31 210.26,326.33 245.27,235.32 231,206 227.07,197.93 220.64,191 213.64,185.29"];
		h1a_h1c_1 -> h1a_1_c		 [pos="e,144.63,48.377 202.85,372.84 210.02,364.05 217.28,353.64 222,343 238.65,305.46 234.43,292.91 238,252 242.11,204.89 239.47,84.953 \
234,78 224.02,65.308 186.4,55.968 154.68,50.144"];
	}
	subgraph cluster_h2a_h2c {
		graph [bb="278,260,422,456",
			label="Host (h2d):h2a",
			lheight=0.21,
			lp="350,444.5",
			lwidth=1.14
		];
		subgraph cluster_h2a_h2c_2 {
			graph [bb="305,268,395,343",
				label="Type: 2",
				lheight=0.21,
				lp="350,331.5",
				lwidth=0.60
			];
			h2a_2_d			 [height=0.5,
				label="Child d",
				pos="350,294",
				width=1.0284];
		}
		h2a_h2c_2		 [height=0.74639,
			label="2\n[Engine: h2c]",
			pos="350,398",
			width=1.7826];
		h2a_h2c_2 -> h2a_2_d		 [pos="e,350,312.3 350,371 350,356.18 350,337.62 350,322.55"];
	}
	subgraph cluster_h3a_h3c {
		graph [bb="430,363,574,456",
			label="Host (h3d):h3a",
			lheight=0.21,
			lp="502,444.5",
			lwidth=1.14
		];
		h3a_h3c_3		 [height=0.74639,
			label="3\n[Engine: h3c]",
			pos="502,398",
			width=1.7826];
	}
	h1a_1_b -> h2a_2_d	 [label="at to\nat \n(Relation 907)",
		lp="303.5,229",
		pos="e,318.76,284.32 207.79,178.13 220.51,184.83 234.95,194.2 245,206 259.34,222.84 248.78,235.95 264,252 276.25,264.92 293.41,274.23 \
309.18,280.67"];
	h2a_2_d -> h1a_1_a	 [label="l to\nl \n(Relation 904)",
		lp="230.48,319.5",
		pos="e,148.23,294 312.85,294 271.48,294 204.36,294 158.3,294"];
	h2a_2_d -> h1a_1_b	 [label="l to\nl \n(Relation 903)",
		lp="397.5,229",
		pos="e,219.23,169.96 354.95,275.85 359.38,256.61 362.97,225.54 347,206 332.24,187.94 273.15,177.01 229.42,171.25"];
}

mre.rb

#!/usr/bin/env ruby
$stdout.sync = true

## Adapted from:
## http://www.omninerd.com/articles/Automating_Data_Visualization_with_Ruby_and_Graphviz
##
require 'ruby-graphviz'
#require 'graphviz/dsl'
require 'awesome_print' # usage #ap ....

# Constants defining file names and paths
pd = ::Pathname.new(__FILE__).dirname
OutputPath          = pd

# expand_path

# load 3 arrays with CSV data
tables = [['1.a','1'],
          ['1.b','2'],
          ['1.c','3'],
          ['2.d','4']]
hosts = [['h1a','1','h1c','h1d'],
         ['h2a','2','h2c','h2d'],
         ['h3a','3','h3c','h3d']]
relations = [
    [900,'aa','1.a','aa','1.b'],
    [901,'at','1.b','at','1.a'],
    [902,'at','1.b','at','1.c'],
    [903,'l','2.d','l','1.b'],
    [904,'l','2.d','l','1.a'],
    [905,'aa','1.c','aa','1.a'],
    [906,'aa','1.c','aa','1.b'],
    [907,'at','1.b','at','2.d'],
    ]

# initialize new Graphviz graph
# g = ::GraphViz::new( 'structs' )
g  = strict('structs')

#g[:rankdir] = 'LR'

# # set global node options
# g.node[:color]    = '#ddaa66'
# g.node[:style]    = 'filled'
# g.node[:shape]    = 'box'
# g.node[:penwidth] = '1'
# g.node[:fontname] = 'Trebuchet MS'
# g.node[:fontsize] = '8'
# g.node[:fillcolor]= '#ffeecc'
# g.node[:fontcolor]= '#775500'
# g.node[:margin]   = '0.0'
#
# # set global edge options
# g.edge[:color]    = '#999999'
# g.edge[:weight]   = '1'
# g.edge[:fontsize] = '6'
# g.edge[:fontcolor]= '#444444'
# g.edge[:fontname] = 'Verdana'
# g.edge[:dir]      = 'forward'
# g.edge[:arrowsize]= '0.5'
#
# draw our clusters of graphs, i.e. hosts
hosts.each do |hosts|
  host = hosts[0]
  schema = hosts[1]
  engine = hosts[2]
  os = hosts[3]
  g.add_graph("cluster_#{host}_#{engine}",:label => "Host (#{os}):" + host) do |gg|
    gg.add_node("#{host}_#{engine}_#{schema}").label = "#{schema}\n[Engine: #{engine}]"
  end
end

# brute force, but simple function to find a type record by schema.table
def find_type(tables_array, material_number)
  tables_array.collect{|m| m[1] unless m.index(material_number).nil?}.compact[0]
end

# brute force, but simple function to find a host record by schema
def find_host(hosts_array, schema_name)
  hosts_array.collect{|m| m[0] unless m.index(schema_name).nil?}.compact[0]
end

# brute force, but simple function to find a engine record by schema
def find_engine(hosts_array, schema_name)
  hosts_array.collect{|m| m[2] unless m.index(schema_name).nil?}.compact[0]
end

# connect the nodes with our recipes, add labels from material data
relations.each do |relation|
  #puts relation.inspect
  ps = relation[2].split('.')[0] # parent schema
  cs = relation[4].split('.')[0] # child schema
  pe = relation[2].split('.')[1] # parent entity
  ce = relation[4].split('.')[1] # child entity
  pf = relation[1] # parent field
  cf = relation[3] # child field
  ct = find_type(tables, relation[2]) # parent type
  pt = find_type(tables, relation[4]) # child type

  ph  = find_host( hosts, ps )
  ch  = find_host( hosts, cs )
  peg = find_engine( hosts, ps )
  ceg = find_engine( hosts, cs )

  ptn_id = "#{ph}_#{ps}_#{pe}" # parent entity (table, etc.) node ID
  ctn_id = "#{ch}_#{cs}_#{ce}" # child entity (table, etc.) node ID
  psn_id = "#{ph}_#{peg}_#{ps}"
  csn_id = "#{ch}_#{ceg}_#{cs}"


  pc = g.get_graph( "cluster_#{ph}_#{peg}" ) # parent host cluster
  cc = g.get_graph( "cluster_#{ch}_#{ceg}" ) # child host cluster

  psn = pc.get_node(psn_id) # Node representing parent schema
  csn = cc.get_node(csn_id) # Node representing child schema

  pcs = pc.add_graph("cluster_#{ph}_#{peg}_#{ps}", :label => "Type: #{pt}")
  ccs = cc.add_graph("cluster_#{ch}_#{ceg}_#{cs}", :label => "Type: #{ct}")


  ccs.add_node( ctn_id, :label=> "Child #{ce}" )
  pcs.add_node( ptn_id, :label=> "Parent #{pe}" )

  g.add_edge(psn_id,ptn_id)
  g.add_edge(csn_id,ctn_id)

  # This works
  g.add_edge(ptn_id, ctn_id).label= "#{pf} to\n#{cf} \n(Relation #{relation[0]})"

end

output = g.output( :dot => OutputPath + 'mre.dot' )

g.output( :svg => OutputPath + 'mre.svg' )

mre.svg