Fix foreign key violations during partition detachments

README.md

@@ -189,6 +189,9 @@ When tables have foreign key relationships, registration order and retention con
 - A child table's `retain_for` must be less than or equal to the parent table's `retain_for`.
   If a child retains partitions longer than its parent, dropping the parent partition will
   fail because the child's foreign key still references it.
+- Foreign keys between the partitioned tables must be declared with `drop_foreign_keys_on_detach`
+  on the model. Otherwise, the foreign key of a detached partition will block detaching partitions
+  of the referenced table.
 
 #### Schema Cleaner Integration
 

lib/code0/zero_track/database/partitioning/partition_manager.rb

@@ -64,6 +64,8 @@ def detach_partitions!
               model.partitioning_strategy.partitions_to_detach.each do |partition|
                 detach_partition(partition, connection)
               end
+
+              drop_declared_foreign_keys_from_detached_partitions(connection)
             end
           end
 
@@ -109,6 +111,73 @@ def detach_partition(partition, connection)
             )
           end
 
+          def drop_declared_foreign_keys_from_detached_partitions(connection)
+            constraint_names = model.foreign_keys_to_drop_on_detach || []
+            return if constraint_names.empty?
+
+            warn_about_unknown_foreign_keys(constraint_names, connection)
+
+            detached_foreign_keys(constraint_names, connection).each do |qualified_table, constraint_name|
+              connection.execute(
+                "ALTER TABLE #{qualified_table} " \
+                "DROP CONSTRAINT #{connection.quote_column_name(constraint_name)}"
+              )
+
+              logger.info(
+                message: 'Dropped foreign key from detached partition',
+                table_name: model.table_name,
+                partition_name: qualified_table,
+                constraint_name: constraint_name
+              )
+            end
+          end
+
+          # We check the parent table because we drop the FK from the partition
+          # when detaching. A missing FK on the detached partition is an expected state
+          def warn_about_unknown_foreign_keys(constraint_names, connection)
+            existing = parent_foreign_key_names(connection)
+            unknown = constraint_names - existing
+            return if unknown.empty?
+
+            logger.warn(
+              message: 'Configured foreign keys to drop on detach do not exist on the partitioned table',
+              table_name: model.table_name,
+              constraint_names: unknown
+            )
+          end
+
+          def parent_foreign_key_names(connection)
+            connection.select_values(<<~SQL.squish)
+              SELECT con.conname
+              FROM pg_catalog.pg_constraint con
+              WHERE con.contype = 'f'
+                AND con.conrelid = #{connection.quote(model.table_name)}::regclass
+            SQL
+          end
+
+          def detached_foreign_keys(constraint_names, connection)
+            schema = Rails.application.config.zero_track.db_partitioning.dynamic_partition_schema
+            quoted_names = constraint_names.map { |name| connection.quote(name) }.join(', ')
+
+            rows = connection.select_rows(<<~SQL.squish)
+              SELECT (quote_ident(n.nspname) || '.' || quote_ident(c.relname)) AS qualified_table,
+                     con.conname AS constraint_name
+              FROM pg_catalog.pg_constraint con
+              JOIN pg_catalog.pg_class c ON c.oid = con.conrelid
+              JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
+              WHERE con.contype = 'f'
+                AND con.conname IN (#{quoted_names})
+                AND n.nspname = #{connection.quote(schema)}
+                AND c.relkind = 'r'
+                AND NOT c.relispartition
+                AND NOT EXISTS (
+                  SELECT 1 FROM pg_catalog.pg_inherits i WHERE i.inhrelid = c.oid
+                )
+            SQL
+
+            rows.map { |qualified_table, constraint_name| [qualified_table, constraint_name] }
+          end
+
           def drop_partition(detached_partition, connection)
             schema_name = connection.quote_table_name(detached_partition.schema)
             partition_name = connection.quote_table_name(detached_partition.name)

lib/code0/zero_track/database/partitioning/partitioned_table.rb

@@ -25,6 +25,16 @@ def partition_by(column, strategy:, **kwargs)
 
               @partitioning_strategy = strategy_class.new(self, column, **kwargs)
             end
+
+            def drop_foreign_keys_on_detach(*constraint_names)
+              @foreign_keys_to_drop_on_detach ||= []
+              @foreign_keys_to_drop_on_detach.concat(constraint_names.flatten.map(&:to_s))
+              @foreign_keys_to_drop_on_detach
+            end
+
+            def foreign_keys_to_drop_on_detach
+              @foreign_keys_to_drop_on_detach || []
+            end
           end
         end
       end

spec/code0/zero_track/database/partitioning/partition_manager_spec.rb

@@ -9,19 +9,26 @@
   let(:model) do
     model = double('Model', table_name: 'events') # rubocop:disable RSpec/VerifiedDoubles
     allow(model).to receive(:try).with(:partitioning_strategy).and_return(partitioning_strategy)
-    allow(model).to receive(:partitioning_strategy).and_return(partitioning_strategy)
+    allow(model).to receive_messages(
+      partitioning_strategy: partitioning_strategy,
+      foreign_keys_to_drop_on_detach: foreign_keys_to_drop_on_detach
+    )
     allow(model).to receive(:with_connection).and_yield(connection)
     model
   end
 
   let(:partitioning_strategy) { double('Strategy') }
 
+  let(:foreign_keys_to_drop_on_detach) { [] }
+
   let(:connection) do
     connection = double('Connection')
     allow(connection).to receive(:execute)
     allow(connection).to receive(:transaction).and_yield
     allow(connection).to receive(:quote_table_name) { |name| "\"#{name}\"" }
+    allow(connection).to receive(:quote_column_name) { |name| "\"#{name}\"" }
     allow(connection).to receive(:quote) { |value| "'#{value}'" }
+    allow(connection).to receive_messages(select_rows: [], select_values: [])
     connection
   end
 
@@ -172,6 +179,105 @@
 
       expect(connection).not_to have_received(:execute).with(a_string_matching(/DETACH/))
     end
+
+    context 'when the model declares foreign keys to drop on detach' do
+      let(:foreign_keys_to_drop_on_detach) { %w[fk_events_parent fk_events_other] }
+
+      let(:partition) do
+        Code0::ZeroTrack::Database::Partitioning::TimePartition.new(
+          model, '2023-01-01', '2023-02-01', partition_name: 'events_202301'
+        )
+      end
+
+      before do
+        allow(partitioning_strategy).to receive(:partitions_to_detach).and_return([partition])
+      end
+
+      it 'drops the declared foreign keys found on detached partitions' do
+        allow(connection).to receive(:select_rows).and_return(
+          [['"partitions_dynamic"."events_202301"', 'fk_events_parent']]
+        )
+
+        described_class.new(model).detach_partitions!
+
+        expect(connection).to have_received(:execute).with(
+          a_string_matching(
+            /ALTER TABLE "partitions_dynamic"."events_202301" DROP CONSTRAINT "fk_events_parent"/
+          )
+        )
+      end
+
+      it 'reconciles every detached partition, not only the one detached in this run' do
+        # Two already-detached partitions still carry the inherited foreign key.
+        allow(connection).to receive(:select_rows).and_return(
+          [
+            ['"partitions_dynamic"."events_202301"', 'fk_events_parent'],
+            ['"partitions_dynamic"."events_202212"', 'fk_events_parent']
+          ]
+        )
+
+        described_class.new(model).detach_partitions!
+
+        expect(connection).to have_received(:execute).with(
+          a_string_matching(/"partitions_dynamic"."events_202301" DROP CONSTRAINT "fk_events_parent"/)
+        )
+        expect(connection).to have_received(:execute).with(
+          a_string_matching(/"partitions_dynamic"."events_202212" DROP CONSTRAINT "fk_events_parent"/)
+        )
+      end
+
+      it 'scopes the catalog lookup to the declared constraint names and dynamic schema' do
+        described_class.new(model).detach_partitions!
+
+        expect(connection).to have_received(:select_rows).with(
+          a_string_matching(/pg_catalog\.pg_constraint.*contype = 'f'/m)
+        )
+        expect(connection).to have_received(:select_rows).with(
+          a_string_matching(/'fk_events_parent', 'fk_events_other'/)
+        )
+        expect(connection).to have_received(:select_rows).with(
+          a_string_matching(/'partitions_dynamic'/)
+        )
+      end
+
+      it 'does not attempt to drop anything when the catalog reports no matching foreign keys' do
+        allow(connection).to receive(:select_rows).and_return([])
+
+        described_class.new(model).detach_partitions!
+
+        expect(connection).not_to have_received(:execute).with(a_string_matching(/DROP CONSTRAINT/))
+      end
+
+      it 'skips the catalog lookup entirely when no foreign keys are declared' do
+        allow(model).to receive(:foreign_keys_to_drop_on_detach).and_return([])
+
+        described_class.new(model).detach_partitions!
+
+        expect(connection).not_to have_received(:select_rows)
+      end
+
+      it 'warns about declared foreign keys missing from the partitioned parent table' do
+        allow(connection).to receive(:select_values).and_return(%w[fk_events_parent])
+
+        described_class.new(model).detach_partitions!
+
+        expect(rails_logger).to have_received(:warn).with(
+          hash_including(
+            message: /do not exist on the partitioned table/,
+            table_name: 'events',
+            constraint_names: %w[fk_events_other]
+          )
+        )
+      end
+
+      it 'does not warn when all declared foreign keys exist on the partitioned parent table' do
+        allow(connection).to receive(:select_values).and_return(%w[fk_events_parent fk_events_other])
+
+        described_class.new(model).detach_partitions!
+
+        expect(rails_logger).not_to have_received(:warn)
+      end
+    end
   end
 
   describe '#drop_partitions' do

spec/code0/zero_track/database/partitioning/partitioned_table_spec.rb

@@ -61,5 +61,30 @@
       end.to raise_error(ArgumentError, /already partitioned/)
     end
   end
+
+  describe '.drop_foreign_keys_on_detach' do
+    it 'defaults to an empty list' do
+      expect(test_class.foreign_keys_to_drop_on_detach).to eq([])
+    end
+
+    it 'records declared constraint names as strings' do
+      test_class.drop_foreign_keys_on_detach(:fk_a, 'fk_b')
+
+      expect(test_class.foreign_keys_to_drop_on_detach).to eq(%w[fk_a fk_b])
+    end
+
+    it 'accumulates across multiple declarations' do
+      test_class.drop_foreign_keys_on_detach(:fk_a)
+      test_class.drop_foreign_keys_on_detach(:fk_b, :fk_c)
+
+      expect(test_class.foreign_keys_to_drop_on_detach).to eq(%w[fk_a fk_b fk_c])
+    end
+
+    it 'flattens an array argument' do
+      test_class.drop_foreign_keys_on_detach(%i[fk_a fk_b])
+
+      expect(test_class.foreign_keys_to_drop_on_detach).to eq(%w[fk_a fk_b])
+    end
+  end
 end
 # rubocop:enable RSpec/VerifiedDoubles