(Somewhat related to my previous post. This came out of the same feature request.)

I hadn’t explored Postgres’s JSON support until recently, and I was, as usual, impressed.

We have an STI model (call it Bucket) in our application with a polymorphic association (call it contents). In the UI, we have an index page that lists Buckets by type with some Bucket-generic information, like volume and material. The new request was to add to that index page data about the contents of the buckets, and to make it searchable and sortable and whatnot. The contents of the bucket can be basically anything, though constant for a given Bucket type, so the UI needed to be flexible enough to render arbitrary columns of data in table and provide some way of searching them. We already use Ransack around the app, so that seemed like the way to go.

class Bucket < ApplicationRecord
  belongs_to :contents, polymorphic: true

  # attribute :type
  # attribute :contents_type
  # attribute :contents_id

  # attribute :volume
  # attribute :material
end

class SolidsBucket < Bucket; end
class LiquidsBucket < Bucket; end

class Solid < ApplicationRecord
  # attribute :name
  # attribute :unique_key

  # attribute :density
  # attribute :hardness
  # attribute :classification
end

class Liquid < ApplicationRecord
  # attribute :name
  # attribute :unique_key

  # attribute :temperature
  # attribute :fluidity
end

SELECT
  s.id,
  s.name,
  s.unique_key,
  'Solid' AS contents_type
FROM solids

UNION ALL

SELECT
  l.id,
  l.name,
  l.unique_key,
  'Liquid' AS contents_type
FROM liquids

-- more contents types...

-- e.g., SELECT ... FROM solids
jsonb_build_object(
  'density', s.density,
  'hardness', s.hardness,
  'classification', s.classification
) AS metadata

CREATE INDEX index_contents_on_metadata ON contents USING GIN (metadata);

SELECT cast(metadata ->> 'density') AS float FROM contents WHERE contents_type = 'Solid';

ransacker :contents_metadata, args: [:parent, :ransacker_args] do |parent, (attribute, type)|
  Arel::Nodes::NamedFunction('cast', [
    Arel::Nodes::InfixOperation.new('->>', parent.table[:metadata], Arel::Nodes.build_quoted(attribute)).as(type)
  ])
end

contents.ransack(
  contents_type_eq: 'Solid',
  c: [{
    a: {
      '0' => {
        name: 'contents_metadata',
        ransacker_args: ['density', 'float']
      }
    },
    p: 'gt',
    v: ['23.2']
  }],
  s: {
    '0' => { name: 'contents_metadata', dir: 'desc', ransacker_args: ['hardness', 'float'] },
    '1' => 'sortable_contents_metadata_(hardness)_(float) desc'
  }
)

module SolidsBucketMetadata
  include Attribute::DSL

  attribute :density, type: :float
  attribute :hardness, type: :float, format: :percentage
  attribute :classification, type: :text, values: %w(Wood Metal Glass Plastic)
end

<input type="hidden" name="q[c][0][a][0][name]" value="contents_metadata">
<input type="hidden" name="q[c][0][a][0][ransacker_args][]" value="density">
<input type="hidden" name="q[c][0][a][0][ransacker_args][]" value="float">
<input type="hidden" name="q[c][0][p]" value="gt">
<input type="search" name="q[c][0][v][]">

I’m not sure exactly which design pattern this is, but I found it useful today.

I had a class that performed a query and returned the results in JSON format for a charting library. The query grouped by one column (column), calculated the average of another column ('metric'), counted the number of records in each group, and did a little formatting of the values before returning the JSON. This is an approximation of what it looked like (lots of other logic omitted for brevity):

ChartData = Struct.new(:scope, :column) do
  def as_json(options = nil)
    values = results.map do |record|
      { x: "#{record.name} (#{record.count})", y: record.score }
    end

    [{ key: column, values: values }]
  end
  
  private

  def results
    scope
      .group(group)
      .select(group.as('name'), count, score)
  end
  
  def group
    scope.arel_table[column]
  end

  def count
    Arel::Nodes::NamedFunction.new('count', [Arel.sql('*')]).as('count')
  end

  def score
    Arel::Nodes::NamedFunction.new('avg', ['metric']).as('score')
  end
end

This worked great. Pretty easy to test, and flexible enough to work with any table that had the appropriate columns.

Then we got a new requirement. We needed to be able to perform the “count” portion of the query on a table that didn’t have the appropriate columns (i.e., it lacked a “metric”), and use the count rather than the score as the Y value of the chart.

At first, it seemed like a matter of reformatting the JSON, but that wasn’t sufficient. The query itself had to change, and that was buried in a private method. It was a big enough change to be painful, but small enough that it didn’t feel like a whole new class.

I first tried passing a new include_scores boolean to the class:

ChartData = Struct.new(:scope, :column, :include_scores) do
  def as_json(options = nil)
    values = results.map do |record|
      if include_scores
        { x: "#{record.name} (#{record.count})", y: record.score }
      else
        { x: record.name, y: record.count }
      end
    end
    
    [{ key: column, values: values }]
  end
  
  private
  
  def results
    if include_scores
      scope
        .group(group)
        .select(group.as('name'), count, score)
    else
      scope
        .group(group)
        .select(group.as('name'), count)
    end
  end
  
  # ...

end

That worked, but it was ugly. The real query was more complex than this, and I didn’t like duplicating all that code. Likewise, the JSON wasn’t two slightly different representations of the same data. It was really two different datas being represented as JSON.

After some more refactoring, I ended up with this:

ChartData = Struct.new(:scope, :column) do
  def scores
    data_points_with_score(average_score) do |record|
      { x: "#{record.name} (#{record.count})", y: record.score }
    end
  end
  
  def counts
    data_points_with_score(null_score) do |record|
      { x: record.name, y: record.count }
    end
  end

  private
  
  def data_points_with_score(score)
    values = scope
      .group(group)
      .select(group.as('name'), count, score)
      .map do |record|
        yield record
      end
    
    [{ key: column, values: values }]
  end
  
  def average_score
    Arel::Nodes::NamedFunction.new('avg', ['metric']).as('score')
  end
  
  def null_score
    Arel.sql('NULL').as('score')
  end
  
  # ...
end

I replaced the single as_json method with two methods, one for each JSON format that I needed to support. I also replaced the single score method with average_score (the actual score calculation) and null_score (effectively a no-op). Since the caller was now responsible for doing the “switching” — i.e., either calling scores or counts — I could declare the scoring method and the JSON format in the same place, and nothing else in the class needed to know about it.

I can see an argument that this is still violating Single Responsibility Principle. Maybe if there were one more case to handle, I would split this into multiple classes. For now, this remains easy to test, and the exceptional behavior is grouped together, so I call it a win.

I just ran across something from the archives of my digital life:

Back in 2010, I recorded a cover of the Angry Video Game Nerd theme song in GarageBand, because that’s what people did way back then. I decided to go jazz, since I had never heard a jazz cover before, and Christmas was coming up, so it felt gentle and snowy to me:

Every note was painstakingly struck using the touchpad on my MacBook Pro (i.e., I’m not a real musician). I’m actually kind of proud of this. It’s the closest I’ve ever come to composing music.

But the timing! This was the year that James managed to get Bear McCreary to compose the music for the Christmas episode, which was a take on Seuss’s Grinch story. So needless to say, my cover…mmm, fell by the wayside.

Fun to go back and listen to this again. I wonder if I could learn some of the parts on an actual (musical) keyboard.