replace=True and ignore=True parameters for create_view(), closes #106

docs/python-api.rst

@@ -879,6 +879,16 @@ The ``.create_view()`` method on the database class can be used to create a view
         select * from dogs where is_good_dog = 1
     """)
 
+This will raise a ``sqlite_utils.utils.OperationalError`` if a view with that name already exists.
+
+You can pass ``ignore=True`` to silently ignore an existing view and do nothing, or ``replace=True`` to replace an existing view with a new definition if your select statement differs from the current view:
+
+.. code-block:: python
+
+    db.create_view("good_dogs", """
+        select * from dogs where is_good_dog = 1
+    """, replace=True)
+
 Storing JSON
 ============
 

sqlite_utils/db.py

@@ -322,14 +322,23 @@ class Database:
             hash_id=hash_id,
         )
 
-    def create_view(self, name, sql):
-        self.conn.execute(
-            """
-            CREATE VIEW {name} AS {sql}
-        """.format(
-                name=name, sql=sql
-            )
-        )
+    def create_view(self, name, sql, ignore=False, replace=False):
+        assert not (
+            ignore and replace
+        ), "Use one or the other of ignore/replace, not both"
+        create_sql = "CREATE VIEW {name} AS {sql}".format(name=name, sql=sql)
+        if ignore or replace:
+            # Does view exist already?
+            if name in self.view_names():
+                if ignore:
+                    return self
+                elif replace:
+                    # If SQL is the same, do nothing
+                    if create_sql == self[name].schema:
+                        return self
+                    self[name].drop()
+        self.conn.execute(create_sql)
+        return self
 
     def m2m_table_candidates(self, table, other_table):
         "Returns potential m2m tables for arguments, based on FKs"

tests/test_create.py

@@ -716,13 +716,6 @@ def test_insert_hash_id(fresh_db):
     assert 1 == dogs.count
 
 
-def test_create_view(fresh_db):
-    fresh_db["data"].insert({"foo": "foo", "bar": "bar"})
-    fresh_db.create_view("bar", "select bar from data")
-    rows = fresh_db.conn.execute("select * from bar").fetchall()
-    assert [("bar",)] == rows
-
-
 def test_vacuum(fresh_db):
     fresh_db["data"].insert({"foo": "foo", "bar": "bar"})
     fresh_db.vacuum()

tests/test_create_view.py

@@ -0,0 +1,43 @@
+import pytest
+from sqlite_utils.utils import OperationalError
+
+
+def test_create_view(fresh_db):
+    fresh_db.create_view("bar", "select 1 + 1")
+    rows = fresh_db.conn.execute("select * from bar").fetchall()
+    assert [(2,)] == rows
+
+
+def test_create_view_error(fresh_db):
+    fresh_db.create_view("bar", "select 1 + 1")
+    with pytest.raises(OperationalError):
+        fresh_db.create_view("bar", "select 1 + 2")
+
+
+def test_create_view_only_arrow_one_param(fresh_db):
+    with pytest.raises(AssertionError):
+        fresh_db.create_view("bar", "select 1 + 2", ignore=True, replace=True)
+
+
+def test_create_view_ignore(fresh_db):
+    fresh_db.create_view("bar", "select 1 + 1").create_view(
+        "bar", "select 1 + 2", ignore=True
+    )
+    rows = fresh_db.conn.execute("select * from bar").fetchall()
+    assert [(2,)] == rows
+
+
+def test_create_view_replace(fresh_db):
+    fresh_db.create_view("bar", "select 1 + 1").create_view(
+        "bar", "select 1 + 2", replace=True
+    )
+    rows = fresh_db.conn.execute("select * from bar").fetchall()
+    assert [(3,)] == rows
+
+
+def test_create_view_replace_with_same_does_nothing(fresh_db):
+    fresh_db.create_view("bar", "select 1 + 1")
+    initial_version = fresh_db.conn.execute("PRAGMA schema_version").fetchone()[0]
+    fresh_db.create_view("bar", "select 1 + 1", replace=True)
+    after_version = fresh_db.conn.execute("PRAGMA schema_version").fetchone()[0]
+    assert after_version == initial_version