Merged in malor/sqlalchemy (pull request #1)

Fix using of 'mysql_length' for composite indexes

lib/sqlalchemy/dialects/mysql/base.py

@@ -226,12 +226,15 @@ become part of the index. SQLAlchemy provides this feature via the
 ``mysql_length`` parameter::
 
     Index('my_index', my_table.c.data, mysql_length=10)
+    Index('a_b_idx', my_table.c.a, my_table.c.b, mysql_length={'a': 4, 'b': 9})
 
 Prefix lengths are given in characters for nonbinary string types and in bytes
-for binary string types. The value passed to the keyword argument will be
-simply passed through to the underlying CREATE INDEX command, so it *must* be
-an integer. MySQL only allows a length for an index if it is for a CHAR,
-VARCHAR, TEXT, BINARY, VARBINARY and BLOB.
+for binary string types. The value passed to the keyword argument *must* be
+either an integer (and, thus, specify the same prefix length value for all
+columns of the index) or a dict in which keys are column names and values are
+prefix length values for corresponding columns. MySQL only allows a length for
+a column of an index if it is for a CHAR, VARCHAR, TEXT, BINARY, VARBINARY and
+BLOB.
 
 Index Types
 ~~~~~~~~~~~~~
@@ -1519,12 +1522,27 @@ class MySQLDDLCompiler(compiler.DDLCompiler):
             text += "UNIQUE "
         text += "INDEX %s ON %s " % (name, table)
 
-        columns = ', '.join(columns)
         if 'mysql_length' in index.kwargs:
             length = index.kwargs['mysql_length']
-            text += "(%s(%d))" % (columns, length)
+
+            # length value can be an integer value specifying the same
+            # prefix length for all columns of the index
+            try:
+                columns = ', '.join(
+                    '%s(%d)' % (col, length)
+                    for col in columns
+                )
+            # otherwise it's a (column_name --> integer value) mapping
+            # specifying the prefix length for each column of the index
+            except TypeError:
+                columns = ', '.join(
+                    ('%s(%d)' % (col, length[col])
+                     if col in length else '%s' % col)
+                    for col in columns
+                )
         else:
-            text += "(%s)" % (columns)
+            columns = ', '.join(columns)
+        text += '(%s)' % columns
 
         if 'mysql_using' in index.kwargs:
             using = index.kwargs['mysql_using']

test/dialect/test_mysql.py

@@ -50,6 +50,35 @@ class CompileTest(fixtures.TestBase, AssertsCompiledSQL):
             'CREATE INDEX test_idx2 ON testtbl (data(5))',
             dialect=mysql.dialect())
 
+    def test_create_composite_index_with_length(self):
+        m = MetaData()
+        tbl = Table('testtbl', m,
+                    Column('a', String(255)),
+                    Column('b', String(255)))
+
+        idx1 = Index('test_idx1', tbl.c.a, tbl.c.b,
+                     mysql_length={'a': 10, 'b': 20})
+        idx2 = Index('test_idx2', tbl.c.a, tbl.c.b,
+                     mysql_length={'a': 15})
+        idx3 = Index('test_idx3', tbl.c.a, tbl.c.b,
+                     mysql_length=30)
+
+        self.assert_compile(
+            schema.CreateIndex(idx1),
+            'CREATE INDEX test_idx1 ON testtbl (a(10), b(20))',
+            dialect=mysql.dialect()
+        )
+        self.assert_compile(
+            schema.CreateIndex(idx2),
+            'CREATE INDEX test_idx2 ON testtbl (a(15), b)',
+            dialect=mysql.dialect()
+        )
+        self.assert_compile(
+            schema.CreateIndex(idx3),
+            'CREATE INDEX test_idx3 ON testtbl (a(30), b(30))',
+            dialect=mysql.dialect()
+        )
+
     def test_create_index_with_using(self):
         m = MetaData()
         tbl = Table('testtbl', m, Column('data', String(255)))