Creating a database that already exists in the glue data catalog raises an exception

moto/glue/exceptions.py

@@ -1,9 +1,15 @@
 from __future__ import unicode_literals
-from moto.core.exceptions import RESTError
+from moto.core.exceptions import JsonRESTError
 
 
-class GlueClientError(RESTError):
+class GlueClientError(JsonRESTError):
+    code = 400
 
-    def __init__(self, *args, **kwargs):
-        kwargs.setdefault('template', 'single_error')
-        super(GlueClientError, self).__init__(*args, **kwargs)
+
+class DatabaseAlreadyExistsException(GlueClientError):
+    def __init__(self):
+        self.code = 400
+        super(DatabaseAlreadyExistsException, self).__init__(
+            'DatabaseAlreadyExistsException',
+            'Database already exists.'
+        )

moto/glue/models.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 from moto.core import BaseBackend, BaseModel
 from moto.compat import OrderedDict
+from.exceptions import DatabaseAlreadyExistsException
 
 
 class GlueBackend(BaseBackend):
@@ -10,6 +11,9 @@ class GlueBackend(BaseBackend):
         self.databases = OrderedDict()
 
     def create_database(self, database_name):
+        if database_name in self.databases:
+            raise DatabaseAlreadyExistsException()
+
         database = FakeDatabase(database_name)
         self.databases[database_name] = database
         return database

tests/test_glue/test_datacatalog.py

@@ -1,7 +1,9 @@
 from __future__ import unicode_literals
 
 import sure  # noqa
+from nose.tools import assert_raises
 import boto3
+from botocore.client import ClientError
 
 from moto import mock_glue
 
@@ -28,3 +30,15 @@ def test_create_database():
     database = response['Database']
 
     database.should.equal({'Name': database_name})
+
+
+@mock_glue
+def test_create_database_already_exists():
+    client = boto3.client('glue', region_name='us-east-1')
+    database_name = 'anewdatabase'
+    create_database(client, database_name)
+
+    with assert_raises(ClientError) as exc:
+        create_database(client, database_name)
+
+    exc.exception.response['Error']['Code'].should.equal('DatabaseAlreadyExistsException')