pg, `change_column_default, :table, :column, nil` issues `DROP DEFAULT`.

Closes #16261.

[Matthew Draper, Yves Senn]

Using `DEFAULT NULL` results in the same behavior as `DROP DEFAULT`.
However, PostgreSQL will cast the default to the columns type,
which leaves us with a default like "default NULL::character varying".

/cc @matthewd

2 files changed, 16 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 43232443e7..f71a887f32 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   `change_column_default :table, :column, nil` with PostgreSQL will issue a
+    `DROP DEFAULT` instead of a `DEFAULT NULL` query.
+
+    Fixes #16261.
+
+    *Matthew Draper*, *Yves Senn*
+
 *   Allow to specify a type for the foreign key column in `references`
     and `add_reference`.
 
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index e09672d239..7042817672 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -423,8 +423,16 @@ module ActiveRecord
         def change_column_default(table_name, column_name, default)
           clear_cache!
           column = column_for(table_name, column_name)
+          return unless column
 
-          execute "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} SET DEFAULT #{quote_default_value(default, column)}" if column
+          alter_column_query = "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} %s"
+          if default.nil?
+            # <tt>DEFAULT NULL</tt> results in the same behavior as <tt>DROP DEFAULT</tt>. However, PostgreSQL will
+            # cast the default to the columns type, which leaves us with a default like "default NULL::character varying".
+            execute alter_column_query % "DROP DEFAULT"
+          else
+            execute alter_column_query % "SET DEFAULT #{quote_default_value(default, column)}"
+          end
         end
 
         def change_column_null(table_name, column_name, null, default = nil)