DEV: column dropper class for cleaner removal of superflous columns

Also fixes issues during deploy cause target column was renamed in
theme_fields

spec/components/column_dropper_spec.rb

@@ -0,0 +1,52 @@
+require 'rails_helper'
+require 'column_dropper'
+
+describe ColumnDropper do
+
+  def has_column?(table, column)
+    Topic.exec_sql("SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS
+                    WHERE
+                      table_schema = 'public' AND
+                      table_name = :table AND
+                      column_name = :column
+                   ",
+                      table: table, column: column
+                  ).to_a.length == 1
+  end
+
+  it "can correctly drop columns after correct delay" do
+    Topic.exec_sql "ALTER TABLE topics ADD COLUMN junk int"
+    name = Topic
+            .exec_sql("SELECT name FROM schema_migration_details LIMIT 1")
+            .getvalue(0,0)
+
+    Topic.exec_sql("UPDATE schema_migration_details SET created_at = :created_at WHERE name = :name",
+                  name: name, created_at: 15.minutes.ago)
+
+    dropped_proc_called = false
+
+    ColumnDropper.drop(
+      table: 'topics',
+      after_migration: name,
+      columns: ['junk'],
+      delay: 20.minutes,
+      on_remove: ->(){dropped_proc_called = true}
+    )
+
+    expect(has_column?('topics', 'junk')).to eq(true)
+    expect(dropped_proc_called).to eq(false)
+
+    ColumnDropper.drop(
+      table: 'topics',
+      after_migration: name,
+      columns: ['junk'],
+      delay: 10.minutes,
+      on_remove: ->(){dropped_proc_called = true}
+    )
+
+    expect(has_column?('topics', 'junk')).to eq(false)
+    expect(dropped_proc_called).to eq(true)
+
+  end
+end
+