how to pluck or select values from a database and return separate arrays for each record in rails

Question

i have a model History(data:text) and the data is an serialized array

I want to render the all of the saved histories as json; however calling the records forces them into 1 array when I need each history to be contained in a separate hash.

When I call:

History.limit(1).pluck(:data)

everything is fine because I get 1 array:

[{"miners"=>{},
  "stats"=>
   {"amtDue"=>422770422,
    "amtPaid"=>0,
    "hash"=>18,
    "identifier"=>"global",
    "invalidShares"=>1,
    "lastHash"=>1515350526,
    "timeout"=>1515350582003,
    "totalHashes"=>7107814,
    "txnCount"=>0,
    "validShares"=>9598},
  "ticker"=>
   {"base"=>"XMR",
    "change"=>"-5.01749443",
    "price"=>"396.34459617",
    "target"=>"USD",
    "volume"=>"24647.97760706"},
  "timestamp"=>1515350523}]

However, when I call 2 or more records they are contained in 1 array which won't work for my needs:

History.limit(2).pluck(:data)

[{"miners"=>{},
  "stats"=>
   {"amtDue"=>422770422,
    "amtPaid"=>0,
    "hash"=>18,
    "identifier"=>"global",
    "invalidShares"=>1,
    "lastHash"=>1515350526,
    "timeout"=>1515350582003,
    "totalHashes"=>7107814,
    "txnCount"=>0,
    "validShares"=>9598},
  "ticker"=>
   {"base"=>"XMR",
    "change"=>"-5.01749443",
    "price"=>"396.34459617",
    "target"=>"USD",
    "volume"=>"24647.97760706"},
  "timestamp"=>1515350523},
 {"miners"=>{},
  "stats"=>
   {"amtDue"=>422770422,
    "amtPaid"=>0,
    "hash"=>4,
    "identifier"=>"global",
    "invalidShares"=>1,
    "lastHash"=>1515353143,
    "timeout"=>1515353179152,
    "totalHashes"=>7125334,
    "txnCount"=>0,
    "validShares"=>9628},
  "ticker"=>
   {"base"=>"XMR",
    "change"=>"3.74386788",
    "price"=>"405.10595848",
    "target"=>"USD",
    "volume"=>"25292.54683583"},
  "timestamp"=>1515353103}]

How do I get data for each history in a separate hash? Here's my controller, I want to render the arrays as json:

def inquiry
@step = inquiry params[:step]
@n = inquiry_params[:n]

if @n.present?
  @history = History.limit(@n).pluck[:data]
  render :json => @history
else
....
end

 def inquiry_params
    accessible = [:step, :n]
    params.permit(accessible)
  end

edit: I'm now calling :

@history = Hash[History.limit(@n).pluck(:data).flatten.map{ |el| [el] }].flatten

I get the desired format; however there is an unwanted "null" that I don't understand where it comes from or how to get rid.

[{"miners"=>["b9aeff80-e16d-4af6-b675-9f218eff8077"],
  "stats"=>
   {"amtDue"=>422770422,
    "amtPaid"=>0,
    "hash"=>28,
    "identifier"=>"global",
    "invalidShares"=>1,
    "lastHash"=>1515435268,
    "timeout"=>1515435354059,
    "totalHashes"=>9370026,
    "txnCount"=>0,
    "validShares"=>11999},
  "ticker"=>
   {"base"=>"XMR",
    "change"=>"1.18426262",
    "price"=>"383.96558958",
    "target"=>"USD",
    "volume"=>"24605.93213606"},
  "timestamp"=>1515435302},
 nil,
 {"miners"=>["b9aeff80-e16d-4af6-b675-9f218eff8077"],
  "stats"=>
   {"amtDue"=>422770422,
    "amtPaid"=>0,
    "hash"=>28,
    "identifier"=>"global",
    "invalidShares"=>1,
    "lastHash"=>1515435467,
    "timeout"=>1515435505244,
    "totalHashes"=>9374886,
    "txnCount"=>0,
    "validShares"=>12005},
  "ticker"=>
   {"base"=>"XMR",
    "change"=>"0.52934940",
    "price"=>"383.31067636",
    "target"=>"USD",
    "volume"=>"24591.95015563"},
  "timestamp"=>1515435422},
 nil]

Answer 1

I'm not sure if I understood you correctly but it's possible to use map like this:

2.5.0 :001 > z = [ {:s=>{:a=>1, :b=>2}}, {:t=>{:c=>3, :d=>4}} ]
   => [{:s=>{:a=>1, :b=>2}}, {:t=>{:c=>3, :d=>4}}] 
2.5.0 :002 > z.map{ |el| [el] }
   => [[{:s=>{:a=>1, :b=>2}}], [{:t=>{:c=>3, :d=>4}}]]

In your case it should then be History.limit(2).pluck(:data).map{ |el| [el] }

EDIT: By calling compact you will have your result-array with all the nil values removed like this:

2.5.0 :003 > x = [{a: {b: 1, c: 2}}, nil]
    => [{:a=>{:b=>1, :c=>2}}, nil]
2.5.0 :004 > x.compact
    => [{:a=>{:b=>1, :c=>2}}]