diff --git a/Gemfile b/Gemfile
index ca6c58d..ac12765 100644
--- a/Gemfile
+++ b/Gemfile
@@ -59,8 +59,10 @@ group :datamapper do
     gem "dm-#{adapter}-adapter", DM_VERSION, SOURCE => "#{DATAMAPPER}/dm-#{adapter}-adapter#{REPO_POSTFIX}"
   end
 
+  gem "dm-migrations", :git => "#{DATAMAPPER}/dm-migrations.git", :branch => "custom_type_migration"
+
   plugins = ENV['PLUGINS'] || ENV['PLUGIN']
-  plugins = plugins.to_s.tr(',', ' ').split.push('dm-migrations').uniq
+  plugins = plugins.to_s.tr(',', ' ').split.uniq
 
   plugins.each do |plugin|
     gem plugin, DM_VERSION, SOURCE => "#{DATAMAPPER}/#{plugin}#{REPO_POSTFIX}"
diff --git a/spec/fixtures/custom.rb b/spec/fixtures/custom.rb
new file mode 100644
index 0000000..98f8cde
--- /dev/null
+++ b/spec/fixtures/custom.rb
@@ -0,0 +1,20 @@
+module DataMapper
+  module TypesFixtures
+
+    class CustomWithoutOptions
+      include ::DataMapper::Resource
+
+      property :id, Serial
+      property :text, Text
+      property :json, Json
+    end
+
+    class CustomWithOptions
+      include ::DataMapper::Resource
+
+      property :id, Serial
+      property :text, Text, :length => 2**24-1
+      property :json, Json, :length => 2**24-1
+    end
+  end
+end
diff --git a/spec/integration/custom_type_migration_spec.rb b/spec/integration/custom_type_migration_spec.rb
new file mode 100644
index 0000000..2032a24
--- /dev/null
+++ b/spec/integration/custom_type_migration_spec.rb
@@ -0,0 +1,66 @@
+require 'spec_helper'
+
+# When dm-migrations' dm-do-adapter runs, Adapter#property_schema_hash is invoked
+# on each property to generate the SQL for it.
+#
+# For Property::Text, type_map[Property::Text] yields a schema of TEXT with no
+# :length property.  When DM encounters a String primitive whose length exceeds
+# the schema's capacity, it auto-adjusts the schema primitive to compensate
+# (i.e. in MySQL, {SHORT,MEDIUM,LONG}TEXT).  Result: MEDIUMTEXT == AWESOME.
+#
+# The case is different for (1) a custom Property derived from (2) a builtin
+# Property whose schema primitive changes based on the Property's size options.
+# For Property::Json, the first type_map[property.class] lookup is nil because
+# custom types can't/don't update Adapter#type_map -- custom properties can't know
+# what model/repository/adapter they're going to be on at definition time, which
+# they would need because the type_map is stored on the adapter *class*.
+#
+# So, the second lookup type_map[property.primitive] kicks in, which for
+# Property::Json is type_map[String].  That in turn yields a schema of VARCHAR
+# with a :length property.  As with Property::Text, when DM encounters a String
+# primitive whose length exceeds the schema's capacity, it auto-adjusts the schema
+# primitive to compensate (i.e. in MySQL, {SHORT,MEDIUM,LONG}TEXT).  However, when
+# dm-migrations encounters any property_schema_hash with a :length option, it
+# automatically appends "(%i)" % length to the SQL statement.  Result:
+# MEDIUMTEXT(123412341234) == entire migration FKD.
+#
+
+require './spec/fixtures/custom'
+
+try_spec do
+
+  describe 'a model with a builtin property whose schema primitive changes according to size options and a custom property based on it' do
+
+    supported_by :postgres, :mysql, :sqlite, :oracle, :sqlserver do # basically all SQL/DO-based
+
+      {
+        ::DataMapper::TypesFixtures::CustomWithoutOptions => "without any specified property options",
+        ::DataMapper::TypesFixtures::CustomWithOptions => "with specified property options that shift the schema primitive",
+      }.each do |klass, desc|
+
+        describe(desc) do
+          let(:model) { klass }
+
+          context 'when migrated' do
+            it 'should use the same property_schema_hash as its logical parent' do
+              proc { model.auto_upgrade! }.should_not raise_error(DataObjects::SQLError)
+
+              adapter = model.repository.adapter
+
+              json_hash = adapter.send(:property_schema_hash, model.json)
+              text_hash = adapter.send(:property_schema_hash, model.text)
+
+              json_hash.delete(:name)
+              text_hash.delete(:name)
+
+              json_hash.should == text_hash
+            end
+          end
+        end
+
+      end # each test
+
+    end # supported_by
+  end # describe: model
+end # try_spec
+