finally, a really straightforward reduce() method which reduces cols

to the minimal set for every test case I can come up with, and
now replaces all the cruft in Mapper._compile_pks() as well as
Join.__init_primary_key().  mappers can now handle aliased selects
and figure out the correct PKs pretty well [ticket:933]

1 files changed, 24 insertions, 12 deletions

diff --git a/lib/sqlalchemy/sql/util.py b/lib/sqlalchemy/sql/util.py
index 0989cb43e..93998c9a9 100644
--- a/lib/sqlalchemy/sql/util.py
+++ b/lib/sqlalchemy/sql/util.py
@@ -52,30 +52,42 @@ def find_columns(clause):
     
     
 def reduce_columns(columns, *clauses):
-    raise NotImplementedError()
+    """given a list of columns, return a 'reduced' set based on natural equivalents.
+
+    the set is reduced to the smallest list of columns which have no natural
+    equivalent present in the list.  A "natural equivalent" means that two columns
+    will ultimately represent the same value because they are related by a foreign key.
+    
+    \*clauses is an optional list of join clauses which will be traversed
+    to further identify columns that are "equivalent".
     
-    # TODO !!!
-    all_proxied_cols = util.Set(chain(*[c.proxy_set for c in columns]))
+    This function is primarily used to determine the most minimal "primary key"
+    from a selectable, by reducing the set of primary key columns present
+    in the the selectable to just those that are not repeated.
+    
+    """
     
     columns = util.Set(columns)
     
-    equivs = {}
+    omit = util.Set()
     for col in columns:
         for fk in col.foreign_keys:
-            if fk.column in all_proxied_cols:
-                for c in columns:
-                    if col.references(c):
-                        equivs[col] = c
+            for c in columns:
+                if c is col:
+                    continue
+                if fk.column.shares_lineage(c):
+                    omit.add(col)
+                    break
     
     if clauses:
         def visit_binary(binary):
-            if binary.operator == operators.eq and binary.left in columns and binary.right in columns:
-                equivs[binary.left] = binary.right
+            cols = columns.difference(omit)
+            if binary.operator == operators.eq and binary.left in cols and binary.right in cols:
+                omit.add(binary.right)
         for clause in clauses:
             visitors.traverse(clause, visit_binary=visit_binary)
     
-    result = util.Set([c for c in columns if c not in equivs])
-    return expression.ColumnSet(result)
+    return expression.ColumnSet(columns.difference(omit))
 
 class ColumnsInClause(visitors.ClauseVisitor):
     """Given a selectable, visit clauses and determine if any columns