def reduce_columns(columns, *clauses, **kw):
"""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".
\**kw may specify 'ignore_nonexistent_tables' to ignore foreign keys
whose tables are not yet configured.
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.
"""
ignore_nonexistent_tables = kw.pop('ignore_nonexistent_tables', False)
columns = util.ordered_column_set(columns)
omit = util.column_set()
for col in columns:
for fk in chain(*[c.foreign_keys for c in col.proxy_set]):
for c in columns:
if c is col:
continue
try:
fk_col = fk.column
except exc.NoReferencedTableError:
if ignore_nonexistent_tables:
continue
else:
raise
if fk_col.shares_lineage(c):
omit.add(col)
break
if clauses:
def visit_binary(binary):
if binary.operator == operators.eq:
cols = util.column_set(
chain(*[c.proxy_set for c in columns.difference(omit)]))
if binary.left in cols and binary.right in cols:
for c in columns:
if c.shares_lineage(binary.right):
omit.add(c)
break
for clause in clauses:
visitors.traverse(clause, {}, {'binary': visit_binary})
return expression.ColumnSet(columns.difference(omit))
def reduce_columns(columns, *clauses):
"""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".
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.OrderedSet(columns)
omit = util.Set()
for col in columns:
for fk in col.foreign_keys:
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:
cols = util.Set(
chain(*[c.proxy_set for c in columns.difference(omit)]))
if binary.left in cols and binary.right in cols:
for c in columns:
if c.shares_lineage(binary.right):
omit.add(c)
break
for clause in clauses:
visitors.traverse(clause, visit_binary=visit_binary)
return expression.ColumnSet(columns.difference(omit))