ActiveRecord has the ActiveRecord::Base.ignored_columns = %w(some columns
to ignore) method to blocklist columns that ActiveRecord loads from the
database. Spelunking the ActiveRecord source, I found no corresponding
method that acts as an allowlist for columns that will be loaded.I found
myself reaching for an allowlist as I started to take advantage of Rails
6's multi-database support. In my context, data from another database that
used to be wrapped in a REST endpoint can now be accessed directly. When
making this direct access, I want to ignore every attribute except the few
I am specifically interested in. Because we’re in a multiple-app
environment, we don’t necessarily have control over columns being added or
dropped from the secondary database during runtime. We don’t want our
application to go down because another team removed an experimental column
from their database.To solve this problem with ignored_columns, we had to
enumerate all the existing columns.
class Dogs < AnimalsBase
self.ignored_columns = %w(some list of columns that can never be exhaustive
because new columns could be added all the time)
end
This suffers from the problem of new columns could be added at any time.
The list of ignored columns has to be constantly tended to ensure we’re
robust against a runtime error of columns going away.What I wanted to reach
for was something like
class Dogs < AnimalsBase
self.allowed_columns = %w(id and only the exact columns needed)
end
With this approach, we’re robust against the other database adding and
removing columns. We still run the risk of encountering runtime errors if
one of our exactly-requested columns goes away. I can’t come up with a way
to mitigate that risk aside from improving communications between teams.I’m
not solid on the name allowed_columns but it was the first thing I reached
for.For the time being, I implemented this with a monkey patch to
load_schema!
ActiveRecord::Base.concerning "AllowedColumns" do
included do
self.allowed_columns = [].freeze
end
module ClassMethods
def allowed_columns
if defined?(@allowed_columns)
@allowed_columns
else
superclass.allowed_columns
end
end
def allowed_columns=(columns)
@allowed_columns = columns.map(&:to_s)
end
# copied from from
https://github.com/rails/rails/blob/2dea8c29c794bec564a2e69ad715d25024e93932/activerecord/lib/active_record/model_schema.rb#L484-L497
def load_schema!
unless table_name
raise ActiveRecord::TableNotSpecified, "#{self} has no table
configured. Set one with #{self}.table_name="
end
@columns_hash =
connection.schema_cache.columns_hash(table_name).except(*ignored_columns)
@columns_hash = @columns_hash.slice(*allowed_columns) if
allowed_columns.present? # This is the additional line
@columns_hash.each do |name, column|
define_attribute(
name,
connection.lookup_cast_type_from_column(column),
default: column.default,
user_provided_default: false
)
end
end
end
end
Is this a feature that others would find useful now that we’re working with
first-class multi-database support?
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