Question

I need to check if the item is in datastore before I update it.

I have 2 lists: UNIQUES = ["B","K","V"] and COUNTS = [5, 10, 3]

This is the model:

class Rep(db.Model):
    mAUTHOR = db.UserProperty(auto_current_user=True)
    mUNIQUE = db.TextProperty()
    mCOUNT = db.IntegerProperty()
    mDATE = db.DateTimeProperty(auto_now_add=True)

This function updates the database:

def write_to_db(S, C):
    REP = Rep(mUNIQUE=S, mCOUNT=C)
    db.put(REP)

Inspired by this page I try:

for i in range(len(UNIQUES)):            
    C_QUERY = Rep.all()
    C_QUERY.filter("mAUTHOR =", user)
    C_QUERY.filter("mUNIQUE =", UNIQUES[i])
    C_RESULT = C_QUERY.fetch(1)
    if C_RESULT:
        C = C_RESULT.mCOUNT + COUNTS[i]
        S = db.Text(UNIQUES[i])
        write_to_db(S,C)
    else:
        C = COUNTS[i]
        S = UNIQUES[i]
        write_to_db(S, C)

But the result is not what I expect. C_RESULT is always empty; and instead of updating, a new record is created. What am I doing wrong? Thanks!

EDIT3: Problem solved

As per David Underhill’s comment I updated the code and it now works.

        if C_RESULT:
            rep=C_RESULT[0]
            rep.mCOUNT+=COUNTS[i]
            rep.put()

EDIT2: Related question

How do I update this query and put it back with updated information?

C_QUERY = Rep.all()
C_QUERY.filter("mAUTHOR =", user)
C_QUERY.filter("mUNIQUE =", UNIQUES[i])
C_RESULT = C_QUERY.fetch(1)                

I want to change mCOUNT and then write it to datastore. How do I this? This looks like the exact same thing they do in this page but I could not make it work. Thanks for your help.

EDIT

I updated the code per David Underhill’s answer. That solved the problem (but the functionality is not right. I am not sure if that should be a different question).

    for i in range(len(UNIQUES)):                        
        C_QUERY = Rep.all()
        C_QUERY.filter("mAUTHOR =", user)
        C_QUERY.filter("mUNIQUE =", UNIQUES[i])
        C_RESULT = C_QUERY.fetch(1)                
        if C_RESULT:
            C = C_RESULT[0].mCOUNT + COUNTS[i]
            S = UNIQUES[i]
            write_to_db(S, C)
        else:
            C = COUNTS[i]
            S = UNIQUES[i]
            write_to_db(S, C)

Answer 1

The problem is that the query is trying to filter mUNIQUE. However, you declared mUNIQUE as a db.TextProperty which is never indexed. As a result, you query never finds any results.

Solution: Change mUNIQUE to a db.StringProperty (which is indexed by default).

You should also consider updating Rep in a transaction – the current code may fail to property add COUNTS[i] if two requests try to update the same entity concurrently.

Also, you can update S = db.Text(UNIQUES[i]) to simply S = UNIQUES[i].