Find an exact object-m2m match in Python -- no SQL.

has_exact_m2m_match.py

"""
Find Exact M2M - Match
=========

Find an exact m2m match for a model.

Just did for fun in response to a stackoverflow question for a data importing issue....

By Yuji Tomita
2/24/2011
"""
def has_exact_m2m_match(match_list, m2m_model, source_model_field, target_model_field):
    """
    Get exact Obj m2m match 
    """
    if isinstance(match_list, QuerySet):
        match_list = [x.id for x in match_list]
        
    results = {}
    match = set(match_list)
    for source, target in \
        m2m_model.objects.filter(size__in=match).values_list(source_model_field, target_model_field):
        # note: we are accessing the auto generated through model for the sizes m2m
        try:
            results[source].append(target)
        except KeyError:
            results[source] = [target]
            
    return bool(filter(lambda x: set(x) == match, results.values()))
    # filter any specific objects that have the exact same size IDs
    # if there is a match, it means an Obj exists with exactly 
    # the sizes you provided to the function, no more.


match_list = [1, 2, 3, 4, 5, 6, ] # id's of a potential m2m object addition
m2m_model = MyModel.my_m2m_field.through

source_model_field = 'mymodel'
target_model_field = 'my_m2m_target'

exists = has_exact_match(match_list, m2m_model, source_model_field, target_model_field)