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Leaving a Channel: Soft Deletion (Part 1)

Right now we have a basic set up with a user and a channel. We can join a channel, but we can't leave it. Let's implement that.

When we enter a channel, we create a membership join table record. We could consider a few options to indicate a user has left a channel:

  • We could simply delete this membership
  • We could add an "active" column to the membership table and toggle this on/off
  • We could soft delete the membership (mark it as deleted using a column) and create new one for new joins

Now let's consider what memberships can tell us:

  • We may want to understand how many times the user has joined the channel
  • We want to know when the user had last joined a channel, when they left the channel, and how long they have been in the channel
  • Something else perhaps?

For the purposes of this exercise, we are going to implement soft deletion as this will help us answer questions like "how many times has this user joined this channel?"

Soft Deletion

Soft deletion can be implemented in a few ways:

  • We can use a column to mark a record as deleted
  • We can use a timestamp to mark a record as deleted
  • We can use a combination of the above

Discard is a library that can be used to implement soft deletion. It is implemented by @jhawthorn at GitHub and is available as a Gem. It provides some extra functionality for models such as:

  • kept scope (equivalent to where(deleted_at: nil))
  • discarded scope (equivalent to where.not(deleted_at: nil))
  • And others: 
      post = Post.first   # => #<Post id: 1, ...>
  post.discard        # => true
  post.discard!       # => Discard::RecordNotDiscarded: Failed to discard the record
  post.discarded?     # => true
  post.undiscarded?   # => false
  post.kept?          # => false
  post.discarded_at   # => 2017-04-18 18:49:49 -0700

It is encouraged to read through that Gem's README to understand the nuance in soft deletion. For now, though, let's add the gem to our Gemfile and run bundle install.

gem 'discard', '~> 1.2'

NOTE: To take advantage of the discarded column, you cannot call destroy or delete on the record. That will actually delete the record (destroy initiates model callbacks, delete doesn't and just executes SQL). Instead you must call discard or discard! (the latter will raise an error if the record is not discarded).

Adding a Discard Column

We now have the gem installed. Let's add a discarded_at column to our memberships table.

Run the following command to generate the migration:

bin/rails generate migration add_discarded_at_to_memberships discarded_at:datetime:index

Rails will parse this command and generate the following migration:

class AddDiscardedAtToMemberships < ActiveRecord::Migration[6.1]
  def change
    add_column :memberships, :discarded_at, :datetime
    add_index :memberships, :discarded_at
  end
end

There are a few new things here:

  • It knows how to parse the table/model name out of the command. In this case it parses Memberships out of add_discarded_at_to_memberships
  • It can accept a :index option to add an index to the column

Now we need to add include Discard::Model to our memberships model. It should now look like this:

class Membership < ApplicationRecord
  include Discard::Model
  belongs_to :user
  belongs_to :channel
end

Scopes

current_user.memberships

By default discarded records are included in calls like Membership.all, current_user.memberships, etc. This is great as it can be nuanced when you want to include everything, but you may want to exclude discarded records.

For example, when we join a channel we want to create a new record when no existing undiscarded record exists, but want to create one otherwise.

To accomplish this, we can open up our ChannelsController again and add a kept scope to the current_user.memberships cal which should now be:

current_user.memberships.kept.find_or_create_by!(channel: @channel)

channel.members

We also want to consider our calls to channel.members.count in the sidebar. We want to exclude discarded records when we count members.

members is an association on channel through memberships, so we can't add a kept scope to that members. There are a few options:

  • We could add a separate has_many for all members and kept members
  • We could add a separate has_many for kept memberships
  • We could add a where clause to the members call (channel.members.where(membership: { discarded_at: nil }))

All things considered, we typically want to avoid having to add where clauses each time - we prefer to add a scope.

In reading the discard README, they also recommend against adding a default scope to your main scope, so in this case let's add a new scope.

Let's start by adding an active_memberships scope to Channel. We can use has_many like before, but give the name active_memberships. Because we are not using a name that matches a column, we also have to provide a class_name. If we added just a class_name, we would have an identical scope to memberships, so we need to add a constraint to that scope: -> { kept }. This will call our kept scope on the memberships table, which is available from the discard gem.

has_many :active_memberships, -> { kept }, class_name: "Membership"

We can then add an active_members scope to mirror our members scope.

has_many :active_members, through: :active_memberships, source: :user

Now we can change the call from channel.members to channel.active_members and we will get the correct result. We also want to change memberships to active_memberships in the member?(user) method we added before.

user.channels

We also need to fix channels on the user model. Similar to channel, we get our association through a membership scope. We can add an active_memberships like before and change channels to use that:

  has_many :active_memberships, -> { kept }, class_name: "Membership"
  has_many :channels, through: :active_memberships

We will want to change current_user.channels to current_user.active_channels in app/views/layouts/application.html.erb.

Action Items

  1. Add the discard gem to your Gemfile: 
    gem 'discard', '~> 1.2'
  2. Run bundle install
  3. Add a discarded_at column to your memberships table: 
    bin/rails generate migration add_discarded_at_to_memberships discarded_at:datetime:index
  4. Run bin/rails db:migrate
  5. Add include Discard::Model to your memberships model: 
    class Membership < ApplicationRecord
  include Discard::Model
  belongs_to :user
  belongs_to :channel
end
  6. Add a kept scope to your memberships in the ChannelsController#show: 
    def show
  current_user.memberships.kept.find_or_create_by!(channel: @channel)
end
  7. Add active_memberships and active_members scopes to your Channel: 
    has_many :active_memberships, -> { kept }, class_name: "Membership"
has_many :active_members, through: :active_memberships, source: :user
  8. Add active_memberships and active_channels scopes to your Channel: 
    has_many :active_memberships, -> { kept }, class_name: "Membership"
has_many :active_channels, through: :active_memberships, source: :channel
  9. In app/views/layouts/application.html.erb, change current_user.channels to current_user.active_channels and channel.members to channel.active_members
  10. Done!

This section in the example app

https://github.com/dcsil/rails-tutorial-example/commit/22df515bce94c856fe485c0556429e2a160948ec

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