diff --git a/addons/sale_mrp/tests/test_sale_mrp_flow.py b/addons/sale_mrp/tests/test_sale_mrp_flow.py
index 8c6fde71db9..2a1af0ec02b 100644
--- a/addons/sale_mrp/tests/test_sale_mrp_flow.py
+++ b/addons/sale_mrp/tests/test_sale_mrp_flow.py
@@ -37,7 +37,8 @@ class TestSaleMrpFlow(common.TransactionCase):
                 p.route_ids.add(r)
             return p.save()
 
-        self.uom_kg = self.UoM.create({
+        self.uom_kg = self.env['uom.uom'].search([('category_id', '=', self.categ_kgm.id), ('uom_type', '=', 'reference')], limit=1)
+        self.uom_kg.write({
             'name': 'Test-KG',
             'category_id': self.categ_kgm.id,
             'factor_inv': 1,
@@ -50,7 +51,8 @@ class TestSaleMrpFlow(common.TransactionCase):
             'uom_type': 'smaller',
             'factor': 1000.0,
             'rounding': 0.001})
-        self.uom_unit = self.UoM.create({
+        self.uom_unit = self.env['uom.uom'].search([('category_id', '=', self.categ_unit.id), ('uom_type', '=', 'reference')], limit=1)
+        self.uom_unit.write({
             'name': 'Test-Unit',
             'category_id': self.categ_unit.id,
             'factor': 1,
diff --git a/addons/stock/tests/common.py b/addons/stock/tests/common.py
index 31a7f78df1d..8c26b60e6f3 100644
--- a/addons/stock/tests/common.py
+++ b/addons/stock/tests/common.py
@@ -45,7 +45,8 @@ class TestStockCommon(common.TransactionCase):
         self.productE = self.ProductObj.create({'name': 'Product E', 'type': 'product'})
 
         # Configure unit of measure.
-        self.uom_kg = self.UomObj.create({
+        self.uom_kg = self.env['uom.uom'].search([('category_id', '=', self.categ_kgm), ('uom_type', '=', 'reference')], limit=1)
+        self.uom_kg.write({
             'name': 'Test-KG',
             'category_id': self.categ_kgm,
             'factor_inv': 1,
@@ -71,7 +72,8 @@ class TestStockCommon(common.TransactionCase):
             'factor': 100000.0,
             'rounding': 0.001})
         # Check Unit
-        self.uom_unit = self.UomObj.create({
+        self.uom_unit = self.env['uom.uom'].search([('category_id', '=', self.categ_unit), ('uom_type', '=', 'reference')], limit=1)
+        self.uom_unit.write({
             'name': 'Test-Unit',
             'category_id': self.categ_unit,
             'factor': 1,
diff --git a/addons/stock_landed_costs/tests/test_stock_landed_costs_rounding.py b/addons/stock_landed_costs/tests/test_stock_landed_costs_rounding.py
index 082d5069ca9..f79d285bd23 100644
--- a/addons/stock_landed_costs/tests/test_stock_landed_costs_rounding.py
+++ b/addons/stock_landed_costs/tests/test_stock_landed_costs_rounding.py
@@ -15,7 +15,8 @@ class TestStockLandedCostsRounding(TestStockLandedCostsCommon):
         self._load('stock_account', 'test', 'stock_valuation_account.xml')
 
         # Define undivisible units
-        product_uom_unit_round_1 = self.env['uom.uom'].create({
+        product_uom_unit_round_1 = self.env.ref('uom.product_uom_unit')
+        product_uom_unit_round_1.write({
             'category_id': self.ref('uom.product_uom_categ_unit'),
             'name': 'Undivisible Unit(s)',
             'factor': 1.0,
diff --git a/addons/uom/data/uom_data.xml b/addons/uom/data/uom_data.xml
index 59a2c41208c..2c5cf503797 100644
--- a/addons/uom/data/uom_data.xml
+++ b/addons/uom/data/uom_data.xml
@@ -51,17 +51,17 @@
             <field name="factor" eval="1000"/>
             <field name="uom_type">smaller</field>
         </record>
+        <record id="product_uom_day" model="uom.uom">
+            <field name="name">Day(s)</field>
+            <field eval="uom_categ_wtime" name="category_id"/>
+            <field name="factor" eval="1.0"/>
+        </record>
         <record id="product_uom_hour" model="uom.uom">
             <field name="name">Hour(s)</field>
             <field eval="uom_categ_wtime" name="category_id"/>
             <field name="factor" eval="8.0"/>
             <field name="uom_type">smaller</field>
         </record>
-        <record id="product_uom_day" model="uom.uom">
-            <field name="name">Day(s)</field>
-            <field eval="uom_categ_wtime" name="category_id"/>
-            <field name="factor" eval="1.0"/>
-        </record>
         <record id="product_uom_ton" model="uom.uom">
             <field name="category_id" ref="product_uom_categ_kgm"/>
             <!-- 'tonne' is the most common spelling in english-speaking countries,
diff --git a/addons/uom/models/uom_uom.py b/addons/uom/models/uom_uom.py
index 9608894e485..0baf13b68fe 100644
--- a/addons/uom/models/uom_uom.py
+++ b/addons/uom/models/uom_uom.py
@@ -2,7 +2,7 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 from odoo import api, fields, tools, models, _
-from odoo.exceptions import UserError
+from odoo.exceptions import UserError, ValidationError
 
 
 class UoMCategory(models.Model):
@@ -66,6 +66,27 @@ class UoM(models.Model):
         if self.uom_type == 'reference':
             self.factor = 1
 
+    @api.constrains('category_id', 'uom_type', 'active')
+    def _check_category_reference_uniqueness(self):
+        """ Force the existence of only one UoM reference per category
+            NOTE: this is a constraint on the all table. This might not be a good practice, but this is
+            not possible to do it in SQL directly.
+        """
+        category_ids = self.mapped('category_id').ids
+        self._cr.execute("""
+            SELECT C.id AS category_id, count(U.id) AS uom_count
+            FROM uom_category C
+            LEFT JOIN uom_uom U ON C.id = U.category_id AND uom_type = 'reference'
+            WHERE C.id IN %s
+                AND U.active = 't'
+            GROUP BY C.id
+        """, (tuple(category_ids),))
+        for uom_data in self._cr.dictfetchall():
+            if uom_data['uom_count'] == 0:
+                raise ValidationError(_("UoM category %s should have a reference unit of measure. If you just created a new category, please record the 'reference' unit first.") % (self.env['uom.category'].browse(uom_data['category_id']).name,))
+            if uom_data['uom_count'] > 1:
+                raise ValidationError(_("UoM category %s should only have one reference unit of measure.") % (self.env['uom.category'].browse(uom_data['category_id']).name,))
+
     @api.model
     def create(self, values):
         if 'factor_inv' in values:
diff --git a/addons/uom/tests/test_uom.py b/addons/uom/tests/test_uom.py
index e2bb332754c..66207639f77 100644
--- a/addons/uom/tests/test_uom.py
+++ b/addons/uom/tests/test_uom.py
@@ -2,6 +2,7 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 from odoo.tests.common import TransactionCase
+from odoo.exceptions import ValidationError
 
 
 class TestUom(TransactionCase):
@@ -47,3 +48,50 @@ class TestUom(TransactionCase):
 
         qty = self.uom_unit._compute_quantity(2, product_uom)
         self.assertEquals(qty, 1, "Converted quantity should be rounded up.")
+
+    def test_30_reference_uniqueness(self):
+        """ Check the uniqueness of the reference UoM in a category """
+        time_category = self.env['uom.category'].search([('measure_type', '=', 'time')], limit=1)
+
+        with self.assertRaises(ValidationError):
+            self.env['uom.uom'].create({
+                'name': 'Second Time Reference',
+                'factor_inv': 1,
+                'uom_type': 'reference',
+                'rounding': 1.0,
+                'category_id': time_category.id
+            })
+
+    def test_40_custom_uom(self):
+        """ A custom UoM is an UoM in a category without measurement type. It should behave like a normal UoM """
+        category = self.env['uom.category'].create({
+            'name': 'Custom UoM category',
+        })
+
+        # create the reference
+        self.env['uom.uom'].create({
+            'name': 'Reference UoM of my category',
+            'factor_inv': 1,
+            'uom_type': 'reference',
+            'rounding': 1.0,
+            'category_id': category.id
+        })
+
+        # we can create another UoM now
+        self.env['uom.uom'].create({
+            'name': 'Bigger UoM of my category',
+            'factor_inv': 42,
+            'uom_type': 'bigger',
+            'rounding': 0.5,
+            'category_id': category.id
+        })
+
+        # we can not create a second reference in custom category
+        with self.assertRaises(ValidationError):
+            self.env['uom.uom'].create({
+                'name': 'Second Time Reference',
+                'factor_inv': 1,
+                'uom_type': 'reference',
+                'rounding': 1.0,
+                'category_id': category.id
+            })