Using group_by but return an array of hashes

Question

I simply want to group cities by their state and from there have the array of the hash key (i.e. State Name) return an array of hash data pertaining to it's cities. Right now I have something like this:

City.all.group_by { |c| c.state.name }

Which will return:

{
  "Illinois": [# < City id: 3, name: "Chicago", state_id: 3 > ],
  "Texas": [# < City id: 2, name: "Houston", state_id: 2 > ],
  "California": [# < City id: 1, name: "Los Angeles", state_id: 1 > ],
  "New York": [# < City id: 4, name: "New York City", state_id: 4 > ]
}

Notice how it returns an array of rails objects. Instead I want to return an array of hashes with certain attributes, like their id and name.

You still have to group the data yourself (in your controller). But you don't have to prepare a huge hash representing the entire JSON result. Instead you have a nice DSL to define your JSON (in your view). — Stefan
– Stefan, Commented Dec 4, 2019 at 17:34

3limin4t0r · Accepted Answer · 2019-12-04 17:09:42Z

2

The reason the grouped values are Rails objects (your models) is due to the fact that you also start with these objects. You can use the attributes method to retrieve the attributes of a model instance as a hash.

The following achieves the result you want:

City.all.group_by { |city| city.state.name }
  .transform_values { |cities| cities.map(&:attributes) }

If you only want specific attributes, use slice instead:

City.all.group_by { |city| city.state.name }
  .transform_values { |cities| cities.map { |city| city.slice(:id, :name) } }

Note that slice will return an ActiveSupport::HashWithIndifferentAccess instance. Which mostly can be used in the same manner as a normal hash, but returns the same value for both hash[:name] and hash['name']. If you rather use a normal hash append a to_hash call after the slice call.

edited Dec 4, 2019 at 17:09

answered Dec 4, 2019 at 16:44

3limin4t0r

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Greg · Accepted Answer · 2019-12-04 16:42:50Z

0

This should be enough for you

City.all.group_by { |c| c.state.name }.map {|k,v| [k, v.attributes] }.to_h

and to select only specified attributes do

v.attributes.slice(:name, :id)

answered Dec 4, 2019 at 16:42

Greg

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1 Comment

3limin4t0r Over a year ago

You currently call attributes with an array as receiver, since each value is an array of cities. Resulting in a NoMethodError. Furthermore your slice(:name, :id) should be slice('name', 'id'), since attributes returns a hash where the keys are strings, not symbols.

Karthick Durai · Accepted Answer · 2022-04-06 22:34:12Z

0

One of the easiest approach is to convert it into json object

City.all.as_json.group_by { |c| c.state.name }

this will fix the issue

answered Apr 6, 2022 at 22:34

Karthick Durai

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