diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d5b018..06b63ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- Activity chart no longer crashes the dashboard on multi-database setups where Solid Queue runs on a different engine than the host app's primary database (e.g. a MySQL primary with a dedicated PostgreSQL queue database via `config.solid_queue.connects_to`). The SQL dialect for the time-bucketing query is now detected from the Solid Queue model's own connection instead of `ActiveRecord::Base`, so a `PG::UndefinedFunction: function unix_timestamp(...) does not exist` error is no longer raised. Single-database and same-engine setups are unaffected. (#42)
+
 ## [2.2.0] - 2026-06-24
 
 ### Added
diff --git a/app/services/solid_queue_monitor/chart_data_service.rb b/app/services/solid_queue_monitor/chart_data_service.rb
index 0866c8e..8c0db2d 100644
--- a/app/services/solid_queue_monitor/chart_data_service.rb
+++ b/app/services/solid_queue_monitor/chart_data_service.rb
@@ -61,7 +61,7 @@ def build_buckets(start_time, bucket_seconds)
     # This works identically on PostgreSQL and SQLite.
     def bucket_counts(model, column, start_time, end_time, interval, exclude_nil: false)
       start_epoch = start_time.to_i
-      expr = bucket_index_expr(column, start_epoch, interval)
+      expr = bucket_index_expr(model, column, start_epoch, interval)
 
       scope = model.where(column => start_time..end_time)
       scope = scope.where.not(column => nil) if exclude_nil
@@ -82,18 +82,23 @@ def fill_buckets(buckets, index_counts)
     # PostgreSQL: CAST((EXTRACT(EPOCH FROM col) - start) / interval AS INTEGER)
     # SQLite:     CAST((CAST(strftime('%s', col) AS INTEGER) - start) / interval AS INTEGER)
     # MySQL:      CAST((UNIX_TIMESTAMP(col) - start) / interval AS SIGNED)
-    def bucket_index_expr(column, start_epoch, interval_seconds)
-      if adapter?('sqlite')
+    #
+    # The adapter is detected from the model's own connection, not
+    # ActiveRecord::Base's. Solid Queue is commonly hosted on a dedicated
+    # database (config.solid_queue.connects_to), which may use a different
+    # engine than the host app's primary connection.
+    def bucket_index_expr(model, column, start_epoch, interval_seconds)
+      if adapter?(model, 'sqlite')
         "CAST((CAST(strftime('%s', #{column}) AS INTEGER) - #{start_epoch}) / #{interval_seconds} AS INTEGER)"
-      elsif adapter?('mysql') || adapter?('trilogy')
+      elsif adapter?(model, 'mysql') || adapter?(model, 'trilogy')
         "FLOOR((UNIX_TIMESTAMP(#{column}) - #{start_epoch}) / #{interval_seconds})"
       else
         "FLOOR((EXTRACT(EPOCH FROM #{column}) - #{start_epoch}) / #{interval_seconds})::integer"
       end
     end
 
-    def adapter?(name)
-      ActiveRecord::Base.connection.adapter_name.downcase.include?(name)
+    def adapter?(model, name)
+      model.connection.adapter_name.downcase.include?(name)
     end
   end
 end
diff --git a/spec/services/solid_queue_monitor/chart_data_service_spec.rb b/spec/services/solid_queue_monitor/chart_data_service_spec.rb
index 94fec5e..a6412e3 100644
--- a/spec/services/solid_queue_monitor/chart_data_service_spec.rb
+++ b/spec/services/solid_queue_monitor/chart_data_service_spec.rb
@@ -134,6 +134,53 @@
     end
   end
 
+  describe 'database adapter detection (multi-database safety)' do
+    let(:service) { described_class.new }
+
+    # Builds the bucket-index SQL for a model whose connection reports the given
+    # adapter, independent of the test database engine.
+    def expr_for(adapter_name)
+      connection = instance_double(ActiveRecord::ConnectionAdapters::AbstractAdapter,
+                                   adapter_name: adapter_name)
+      model = class_double(SolidQueue::Job, connection: connection)
+      service.send(:bucket_index_expr, model, 'created_at', 1000, 3600)
+    end
+
+    it 'uses PostgreSQL EXTRACT(EPOCH ...) syntax for a Postgres connection' do
+      expect(expr_for('PostgreSQL'))
+        .to eq('FLOOR((EXTRACT(EPOCH FROM created_at) - 1000) / 3600)::integer')
+    end
+
+    it 'uses MySQL UNIX_TIMESTAMP syntax for a MySQL connection' do
+      expect(expr_for('Mysql2')).to include('UNIX_TIMESTAMP(created_at)')
+    end
+
+    it 'uses MySQL syntax for a Trilogy connection' do
+      expect(expr_for('Trilogy')).to include('UNIX_TIMESTAMP(created_at)')
+    end
+
+    it 'uses SQLite strftime syntax for a SQLite connection' do
+      expect(expr_for('SQLite')).to include("strftime('%s', created_at)")
+    end
+
+    context 'when Solid Queue runs on a different engine than the primary database' do
+      # Regression for issue #42: MySQL primary + dedicated PostgreSQL queue
+      # database (config.solid_queue.connects_to). The dialect must follow the
+      # Solid Queue model's connection, not ActiveRecord::Base's.
+      before do
+        primary = instance_double(ActiveRecord::ConnectionAdapters::AbstractAdapter,
+                                  adapter_name: 'Mysql2')
+        allow(ActiveRecord::Base).to receive(:connection).and_return(primary)
+      end
+
+      it 'builds Postgres SQL when the queue model is on Postgres' do
+        expr = expr_for('PostgreSQL')
+        expect(expr).to include('EXTRACT(EPOCH FROM created_at)')
+        expect(expr).not_to include('UNIX_TIMESTAMP')
+      end
+    end
+  end
+
   describe 'constants' do
     it 'defines all time ranges' do
       expect(described_class::TIME_RANGES.keys).to eq(%w[15m 30m 1h 3h 6h 12h 1d 3d 1w])