human
Solve the following python programming problem given the following code:
def read_fields(self, limit = 0) : 
    keys = [] 
    probes = { 
        
} 
    def probe_record(record, parent = None) : 
        for (key, value) in record.items () : 
            if parent : 
                full_key = ((parent + '.') + key) 
else : 
                full_key = key 
if (self.expand and (type (value) == dict)) : 
                probe_record (value, full_key) 
                continue 
if (not (full_key in probes)) : 
                probe = dq.FieldTypeProbe (full_key) 
                probes [full_key] = probe 
                keys.append (full_key) 
else : 
                probe = probes [full_key] 
probe.probe (value) 
for record in self.document_type.find (limit = limit) : 
        probe_record (record) 
fields = [] 
    for key in keys : 
        probe = probes [key] 
        field = base.Field (probe.field) 
        storage_type = probe.unique_storage_type 
        if (not storage_type) : 
            field.storage_type = 'unknown' 
else : 
            if (storage_type == 'unicode') : 
                field.storage_type = 'string' 
else : 
                field.storage_type = 'unknown' 
                field.concrete_storage_type = fields 
fields.append (field) 
self.fields = list (fields) 
    return self.fields 

    
    

    Fix the buggy line: field.concrete_storage_type = fields
assistant
#Here is Python program to find (limit = limit) : 
.

field.concrete_storage_type = storage_type