Add change nullability statement

spec/avram/migrator/alter_table_statement_spec.cr

@@ -89,6 +89,27 @@ describe Avram::Migrator::AlterTableStatement do
     built.statements[2].should eq "ALTER TABLE ONLY test_defaults ALTER COLUMN money SET DEFAULT '29.99';"
   end
 
+  it "changes defaults after changing the type" do
+    built = Avram::Migrator::AlterTableStatement.new(:users).build do
+      change_type id : Int64
+      change_default id : Int64, default: 54
+    end
+    built.statements.size.should eq 2
+    built.statements[0].should eq "ALTER TABLE users ALTER COLUMN id SET DATA TYPE bigint;"
+    built.statements[1].should eq "ALTER TABLE ONLY users ALTER COLUMN id SET DEFAULT '54';"
+  end
+
+  it "can change column nullability" do
+    built = Avram::Migrator::AlterTableStatement.new(:users).build do
+      forbid_nulls_for :id
+      allow_nulls_for :age
+    end
+
+    built.statements.size.should eq 2
+    built.statements[0].should eq "ALTER TABLE users ALTER COLUMN id SET NOT NULL;"
+    built.statements[1].should eq "ALTER TABLE users ALTER COLUMN age DROP NOT NULL;"
+  end
+
   describe "fill_existing_with" do
     it "fills existing with value and sets column to be non-null for non-null types" do
       built = Avram::Migrator::AlterTableStatement.new(:users).build do

src/avram/migrator/alter_table_statement.cr

@@ -11,6 +11,7 @@ class Avram::Migrator::AlterTableStatement
   getter fill_existing_with_statements = [] of String
   getter change_type_statements = [] of String
   getter change_default_statements = [] of String
+  getter change_nullability_statements = [] of String
   private getter? if_exists : Bool = false
 
   def initialize(@table_name : TableName, *, @if_exists : Bool = false)
@@ -80,6 +81,30 @@ class Avram::Migrator::AlterTableStatement
     add_change_default_statement %column
   end
 
+  # Change the column's nullability from whatever it is currently to true.
+  # ```
+  # alter table_for(User) do
+  #   allow_nulls_for :email
+  # end
+  # ```
+  macro allow_nulls_for(column_name)
+    change_nullability_statements << build_nullability_statement({{column_name.id.stringify}}, true)
+  end
+
+  # Change the column's nullability from whatever it is currently to false.
+  # ```
+  # alter table_for(User) do
+  #   forbid_nulls_for :email
+  # end
+  # ```
+  macro forbid_nulls_for(column_name)
+    change_nullability_statements  << build_nullability_statement({{column_name.id.stringify}}, false)
+  end
+
+  def build_nullability_statement(column_name, nullability)
+    "ALTER TABLE #{@table_name} ALTER COLUMN #{column_name} #{nullability ? "DROP" : "SET"} NOT NULL;"
+  end
+
   def add_change_type_statement(column : ::Avram::Migrator::Columns::Base)
     change_type_statements << column.build_change_type_statement(@table_name)
   end
@@ -114,7 +139,7 @@ class Avram::Migrator::AlterTableStatement
   end
 
   def statements
-    alter_statements + change_default_statements + change_type_statements + index_statements + fill_existing_with_statements
+    alter_statements + change_type_statements + change_default_statements + change_nullability_statements + index_statements + fill_existing_with_statements
   end
 
   def if_exists_statement