diff options
Diffstat (limited to 'lib/sqlalchemy/sql/util.py')
| -rw-r--r-- | lib/sqlalchemy/sql/util.py | 36 |
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 |