An explain query does not raise the `ReadOnlyError` if preventing writes

4 files changed, 11 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
index 15e07229e8..b2545750f4 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb
@@ -19,7 +19,7 @@ module ActiveRecord
           execute(sql, name).to_a
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb
index 0fd14fac0e..a0cac07491 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb
@@ -67,7 +67,7 @@ module ActiveRecord
           end
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :savepoint, :rollback) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :set, :show, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 0871cc5fb1..cb837d0002 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -209,7 +209,7 @@ module ActiveRecord
       # DATABASE STATEMENTS ======================================
       #++
 
-      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :pragma, :savepoint, :rollback) # :nodoc:
+      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :explain, :select, :pragma, :savepoint, :rollback) # :nodoc:
       private_constant :READ_QUERY
 
       def write_query?(sql) # :nodoc:
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index 11df74ea25..30e8a5f833 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1530,4 +1530,12 @@ class BasicsTest < ActiveRecord::TestCase
       assert_equal bird, Bird.where(name: "Bluejay").first
     end
   end
+
+  test "an explain query does not raise if preventing writes" do
+    Bird.create!(name: "Bluejay")
+
+    ActiveRecord::Base.connection.while_preventing_writes do
+      assert_queries(2) { Bird.where(name: "Bluejay").explain }
+    end
+  end
 end