ruby merge array of hashes based on the key and order it by number of key/value pair

Question

I have this array of hashes:

ah = [{"Date"=>"2014-03-17", "countdown 7"=>1}, {"Date"=>"2014-03-17", "voice 6"=>1},     
 {"Date"=>"2014-03-18", "voice 1"=>1}, {"Date"=>"2014-03-18", "voice 2"=>0},     
 {"Date"=>"2014-03-18", "voice 3"=>1}, {"Date"=>"2014-03-18", "voice 4"=>0},    
 {"Date"=>"2014-03-18", "voice 5"=>0}, {"Date"=>"2014-03-18", "voice 6"=>0},     
 {"Date"=>"2014-03-19", "voice 5"=>0}, {"Date"=>"2014-03-19", "voice 6"=>0},    
 {"Date"=>"2014-03-20", "countdown 5"=>1}, {"Date"=>"2014-03-20", "voice 7"=>0},     
 {"Date"=>"2014-03-20", "voice 6"=>0}]   

and i want to merge it based on the key to have :

ah = [{"Date"=>"2014-03-17", "countdown 7"=>1, "voice 6"=>1},      
 {"Date"=>"2014-03-18", "voice 1"=>1, "voice 2"=>0, "voice 3"=>1, "voice 4"=>0, "voice 5"=>0, "voice 6"=>0},       
 {"Date"=>"2014-03-19", "voice 5"=>0, "voice 6"=>0},       
 {"Date"=>"2014-03-20", "countdown 5"=>1, "voice 7"=>0, "voice 6"=>0}]         

Tried :

ah.inject { |all, h| all.merge(h) } #no success

Any hints on how to do that?

update
Would it be possible to sort it by number of key/value pair? So that the hash that have the most key/value be the first and the one that has the least key value be last?

Output

[{"Date"=>"2014-03-18", "voice 1"=>1, "voice 2"=>0, "voice 3"=>1, "voice 4"=>0, "voice 5"=>0, "voice 6"=>0},   
 {"Date"=>"2014-03-20", "countdown 5"=>1, "voice 7"=>0, "voice 6"=>0},
 {"Date"=>"2014-03-17", "countdown 7"=>1, "voice 6"=>1},              
 {"Date"=>"2014-03-19", "voice 5"=>0, "voice 6"=>0}       
]       

** Update 2 **
To sort the array of hashes by lenght of key/value pair:

ah.sort {|a, b| a.length <=> b.length}.reverse  

Answer 1

I'd do :

ah.group_by { |h| h['Date'] }.map { |_,v| v.inject(:update) }
# => [{"Date"=>"2014-03-17", "countdown 7"=>1, "voice 6"=>1},
#     {"Date"=>"2014-03-18",
#      "voice 1"=>1,
#      "voice 2"=>0,
#      "voice 3"=>1,
#      "voice 4"=>0,
#      "voice 5"=>0,
#      "voice 6"=>0},
#     {"Date"=>"2014-03-19", "voice 5"=>0, "voice 6"=>0},
#     {"Date"=>"2014-03-20", "countdown 5"=>1, "voice 7"=>0, "voice 6"=>0}]