From 94bc6e09c0f448fbab2047304f39d4b706abadb5 Mon Sep 17 00:00:00 2001
From: Paul Fitzpatrick <paul.michael.fitzpatrick@gmail.com>
Date: Fri, 21 Oct 2016 16:59:17 -0400
Subject: [PATCH] add Table.create_column_by_example method

This adds a method to create a column by giving an example of
the data it will contain rather than by specifying its
SQLAlchemy type.  This extends the range of operations that
can be performed without reading the SQLAlchemy docs.
---
 dataset/persistence/table.py | 11 +++++++++++
 test/test_persistence.py     | 14 +++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/dataset/persistence/table.py b/dataset/persistence/table.py
index a36876f..4da10ba 100644
--- a/dataset/persistence/table.py
+++ b/dataset/persistence/table.py
@@ -310,6 +310,17 @@ class Table(object):
         finally:
             self.database._release()
 
+    def create_column_by_example(self, name, value):
+        """
+        Explicitly create a new column ``name`` with a type that is appropriate to store
+        the given example ``value``.  The type is guessed in the same way as for the
+        insert method with ``ensure=True``. If a column of the same name already exists,
+        no action is taken, even if it is not of the type we would have created.
+
+            table.create_column_by_example('length', 4.2)
+        """
+        self._ensure_columns({name: value}, {})
+
     def drop_column(self, name):
         """
         Drop the column ``name``.
diff --git a/test/test_persistence.py b/test/test_persistence.py
index 70924a7..11131d9 100644
--- a/test/test_persistence.py
+++ b/test/test_persistence.py
@@ -7,6 +7,7 @@ try:
 except ImportError:  # pragma: no cover
     from ordereddict import OrderedDict  # Python < 2.7 drop-in
 
+from sqlalchemy import FLOAT, INTEGER, TEXT
 from sqlalchemy.exc import IntegrityError, SQLAlchemyError
 
 from dataset import connect
@@ -373,13 +374,24 @@ class TableTestCase(unittest.TestCase):
         assert m['temperature'] == -10, 'new temp. should be -10 but is %d' % m['temperature']
 
     def test_create_column(self):
-        from sqlalchemy import FLOAT
         tbl = self.tbl
         tbl.create_column('foo', FLOAT)
         assert 'foo' in tbl.table.c, tbl.table.c
         assert isinstance(tbl.table.c['foo'].type, FLOAT), tbl.table.c['foo'].type
         assert 'foo' in tbl.columns, tbl.columns
 
+    def test_ensure_column(self):
+        tbl = self.tbl
+        tbl.create_column_by_example('foo', 0.1)
+        assert 'foo' in tbl.table.c, tbl.table.c
+        assert isinstance(tbl.table.c['foo'].type, FLOAT), tbl.table.c['bar'].type
+        tbl.create_column_by_example('bar', 1)
+        assert 'bar' in tbl.table.c, tbl.table.c
+        assert isinstance(tbl.table.c['bar'].type, INTEGER), tbl.table.c['bar'].type
+        tbl.create_column_by_example('pippo', 'test')
+        assert 'pippo' in tbl.table.c, tbl.table.c
+        assert isinstance(tbl.table.c['pippo'].type, TEXT), tbl.table.c['pippo'].type
+
     def test_key_order(self):
         res = self.db.query('SELECT temperature, place FROM weather LIMIT 1')
         keys = list(res.next().keys())