Add rename and rename_belongs_to for migrations.

db/migrations/20200513065134_test_rename_columns.cr

@@ -0,0 +1,27 @@
+class TestRenameColumns::V20200513065134 < Avram::Migrator::Migration::V1
+  def migrate
+    create table_for(RenamableOwner) do
+      primary_key id : Int64
+      add_timestamps
+    end
+
+    create table_for(Renamable) do
+      primary_key id : Int64
+      add_timestamps
+      add thingies : JSON::Any
+      add gsm_number : String
+      add_belongs_to boss : RenamableOwner, on_delete: :cascade
+    end
+
+    alter table_for(Renamable) do
+      rename :gsm_number, :mobile
+      rename_belongs_to :boss, :owner
+      rename :thingies, :options
+    end
+  end
+
+  def rollback
+    drop table_for(Renamable)
+    drop table_for(RenamableOwner)
+  end
+end

spec/migrator/alter_table_statement_spec.cr

@@ -16,11 +16,18 @@ describe Avram::Migrator::AlterTableStatement do
       add future_time : Time, default: Time.local
       add new_id : UUID, default: UUID.new("46d9b2f0-0718-4d4c-a5a1-5af81d5b11e0")
       add numbers : Array(Int32), fill_existing_with: [1]
+      rename :old_name, :new_name
+      rename_belongs_to :owner, :boss
       remove :old_column
       remove_belongs_to :employee
     end
 
-    built.statements.first.should eq <<-SQL
+    built.statements.size.should eq 11
+
+    built.statements[0].should eq "ALTER TABLE users RENAME COLUMN old_name TO new_name;"
+    built.statements[1].should eq "ALTER TABLE users RENAME COLUMN owner_id TO boss_id;"
+
+    built.statements[2].should eq <<-SQL
     ALTER TABLE users
       ADD name text,
       ADD email text,
@@ -39,13 +46,12 @@ describe Avram::Migrator::AlterTableStatement do
       DROP employee_id;
     SQL
 
-    built.statements.size.should eq 9
-    built.statements[1].should eq "CREATE UNIQUE INDEX users_age_index ON users USING btree (age);"
-    built.statements[2].should eq "CREATE INDEX users_num_index ON users USING btree (num);"
-    built.statements[3].should eq "UPDATE users SET email = '[email protected]';"
-    built.statements[4].should eq "ALTER TABLE users ALTER COLUMN email SET NOT NULL;"
-    built.statements[5].should eq "UPDATE users SET updated_at = NOW();"
-    built.statements[6].should eq "ALTER TABLE users ALTER COLUMN updated_at SET NOT NULL;"
+    built.statements[3].should eq "CREATE UNIQUE INDEX users_age_index ON users USING btree (age);"
+    built.statements[4].should eq "CREATE INDEX users_num_index ON users USING btree (num);"
+    built.statements[5].should eq "UPDATE users SET email = '[email protected]';"
+    built.statements[6].should eq "ALTER TABLE users ALTER COLUMN email SET NOT NULL;"
+    built.statements[7].should eq "UPDATE users SET updated_at = NOW();"
+    built.statements[8].should eq "ALTER TABLE users ALTER COLUMN updated_at SET NOT NULL;"
   end
 
   it "does not build statements if nothing is altered" do

src/avram/migrator/alter_table_statement.cr

@@ -6,6 +6,7 @@ class Avram::Migrator::AlterTableStatement
   include Avram::Migrator::MissingOnDeleteWithBelongsToError
 
   getter rows = [] of String
+  getter renamed_rows = [] of String
   getter dropped_rows = [] of String
   getter fill_existing_with_statements = [] of String
   getter change_type_statements = [] of String
@@ -60,17 +61,18 @@ class Avram::Migrator::AlterTableStatement
   end
 
   def alter_statements : Array(String)
-    if (rows + dropped_rows).empty?
-      [] of String
-    else
-      statement = String.build do |statement|
+    alterations = renamed_rows.map do |statement|
+      "ALTER TABLE #{@table_name} #{statement};"
+    end
+    unless (rows + dropped_rows).empty?
+      alterations << String.build do |statement|
         statement << "ALTER TABLE #{@table_name}"
         statement << "\n"
         statement << (rows + dropped_rows).join(",\n")
         statement << ';'
       end
-      [statement]
     end
+    alterations
   end
 
   # Adds a references column and index given a model class and references option.
@@ -166,13 +168,36 @@ class Avram::Migrator::AlterTableStatement
     ]
   end
 
-  def remove(name : Symbol)
-    dropped_rows << "  DROP #{name.to_s}"
+  {% symbol_expected_message = "%s expected a symbol like ':user', instead got: '%s'" %}
+
+  macro rename(old_name, new_name)
+    {% for name in {old_name, new_name} %}
+      {% unless name.is_a?(SymbolLiteral) %}
+        {% raise symbol_expected_message % {"rename", name} %}
+      {% end %}
+    {% end %}
+    renamed_rows << "RENAME COLUMN #{{{old_name}}} TO #{{{new_name}}}"
+  end
+
+  macro rename_belongs_to(old_association_name, new_association_name)
+    {% for association_name in {old_association_name, new_association_name} %}
+      {% unless association_name.is_a?(SymbolLiteral) %}
+        {% raise symbol_expected_message % {"rename_belongs_to", association_name} %}
+      {% end %}
+    {% end %}
+    rename {{old_association_name}}_id, {{new_association_name}}_id
+  end
+
+  macro remove(name)
+    {% unless name.is_a?(SymbolLiteral) %}
+      {% raise symbol_expected_message % {"remove", name} %}
+    {% end %}
+    dropped_rows << "  DROP #{{{name}}}"
   end
 
   macro remove_belongs_to(association_name)
     {% unless association_name.is_a?(SymbolLiteral) %}
-      {% raise "remove_belongs_to expected a symbol like ':user', instead got: '#{association_name}'" %}
+      {% raise symbol_expected_message % {"remove_belongs_to", association_name} %}
     {% end %}
     remove {{ association_name }}_id
   end