Convert Array#select to active record query in rails 4

问题

I'm writing a custom search function, and I have to filter through an association. I have 2 active record backed models, cards and colors with a has_many_and_belongs_to, and colors have an attribute color_name

As my DB has grown to around 10k cards, my search function gets exceptionally slow because i have a select statement with a query inside it, so essentially im having to make thousands of queries.

i need to convert the array#select method into an active record query, that will yield the same results, and im having trouble coming up with a solution. the current (relevant code) is the following:

colors = [['Black'], ['Blue', 'Black']] #this is a parameter retrieved from a form submission
if color
  cards = color.flat_map do |col|
    col.inject( Card.includes(:colors) ) do |memo, color|
        temp = cards.joins(:colors).where(colors: {color_name: color})
        memo + temp.select{|card| card.colors.pluck(:color_name).sort == col.sort}
    end
  end
end

the functionality im trying to mimic is that only cards with colors exactly matching the incoming array will be selected by the search (comparing two arrays). Because cards can be mono-red, red-blue, or red-blue-green etc, i need to be able to search for only red-blue cards or only mono-red cards

I initially started along this route, but i'm having trouble comparing arrays with an active record query

color_objects = Color.where(color_name: col)
Card.includes(:colors).where('colors = ?', color_objects)

returns the error

ActiveRecord::StatementInvalid: PG::SyntaxError: ERROR: syntax error at or near "SELECT" LINE 1: ...id" WHERE "cards"."id" IN (2, 3, 4) AND (colors = SELECT "co...

it looks to me like its failing because it doesnt want to compare arrays, only table elements. is this functionality even possible?

One solution might be to convert the habtm into has many through relation and make join tables which contain keys for every permutation of colors in order to access those directly

回答1:

I need to be able to search for only green-black cards, and not have mono-green, or green-black-red cards show up.

I've deleted my previos answer, because i did not realized you are looking for the exact match.

I played a little with it and i can't see any solution without using an aggregate function. For Postgres it will be array_agg.

You need to generate an SQL Query like:

SELECT *,  array_to_string(array_agg(colors.color_name), ',')) as color_names FROM cards
JOINS cards_colors, colors 
ON (cards.id = cards_colors.card_id AND colors.id = cards_colors.color_id)
GROUP BY cards.id HAVING  color_names = 'green, black' 

I never used those aggregators, so perhaps array_to_string is a wrong formatter, anyway you have to watch for aggregating the colors in alphabetical order. As long as you aint't having too many cards it will be slow enough, but it will scan every card in a table.

I you want to use an index on this query, you should denormalize your data structure, use an array of color_names on a cards record, index that array field and search on it. You can also keep you normalized structure and define an automatic association callback which will put the colorname to the card's color_names array every time a color is assigned to a card.

回答2:

try this

colors = Color.where(color_name: col).pluck(:id)
Card.includes(:colors).where('colors.id'=> colors)

来源：https://stackoverflow.com/questions/24215170/convert-arrayselect-to-active-record-query-in-rails-4