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@@ -260,6 +260,11 @@ module Sequel
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       def set_column_type(name, type)
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         @operations << {:op => :set_column_type, :name => name, :type => type}
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       end
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+      
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+      # Modify a column's NOT NULL constraint.
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+      def set_column_accept_null(name, accept_null)
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+        @operations << {:op => :set_column_null, :name => name, :null => accept_null}
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+      end
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       private
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@@ -33,6 +33,8 @@ module Sequel
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           "ALTER COLUMN #{quoted_name} TYPE #{op[:type]}"
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         when :set_column_default
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           "ALTER COLUMN #{quoted_name} SET DEFAULT #{literal(op[:default])}"
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+        when :set_column_null
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+          "ALTER COLUMN #{quoted_name} #{op[:null] ? 'DROP' : 'SET'} NOT NULL"
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         when :add_index
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           return index_definition_sql(table, op)
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         when :drop_index

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@@ -459,6 +459,20 @@ context "DB#alter_table" do
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   setup do
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     @db = SchemaDummyDatabase.new
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   end
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+  
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+  specify "should allow adding not null constraint" do
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+    @db.alter_table(:cats) do
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+      set_column_accept_null :score, false
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+    end
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+    @db.sqls.should == ["ALTER TABLE cats ALTER COLUMN score SET NOT NULL"]
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+  end
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+  
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+  specify "should allow droping not null constraint" do
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+    @db.alter_table(:cats) do
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+      set_column_accept_null :score, true
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+    end
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+    @db.sqls.should == ["ALTER TABLE cats ALTER COLUMN score DROP NOT NULL"]
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+  end
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   specify "should support add_column" do
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     @db.alter_table(:cats) do