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POHow do I ensure data integrity for objects in google app engine without using key names?
 

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  1. POHow do I ensure data integrity for objects in google app engine without using key names?
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    <p>I'm having a bit of trouble in Google App Engine ensuring that my data is correct when using an ancestor relationship without key names.</p> <p>Let me explain a little more: I've got a parent entity <strong>category</strong>, and I want to create a child entity <strong>item</strong>. I'd like to create a function that takes a category name and item name, and creates both entities if they don't exist. Initially I created one transaction and created both in the transaction if needed using a key name, and this worked fine. However, I realized I didn't want to use the name as the key as it may need to change, and I tried within my transaction to do this:</p> <pre><code>def add_item_txn(category_name, item_name): category_query = db.GqlQuery("SELECT * FROM Category WHERE name=:category_name", category_name=category_name) category = category_query.get() if not category: category = Category(name=category_name, count=0) item_query = db.GqlQuery("SELECT * FROM Item WHERE name=:name AND ANCESTOR IS :category", name=item_name, category=category) item_results = item_query.fetch(1) if len(item_results) == 0: item = Item(parent=category, name=name) db.run_in_transaction(add_item_txn, "foo", "bar") </code></pre> <p>What I found when I tried to run this is that App Engine rejects this as it won't let you run a query in a transaction: <code>Only ancestor queries are allowed inside transactions</code>.</p> <p>Looking at the <a href="http://code.google.com/appengine/docs/python/datastore/functions.html#run_in_transaction" rel="nofollow noreferrer">example</a> Google gives about how to address this:</p> <pre><code>def decrement(key, amount=1): counter = db.get(key) counter.count -= amount if counter.count &lt; 0: # don't let the counter go negative raise db.Rollback() db.put(counter) q = db.GqlQuery("SELECT * FROM Counter WHERE name = :1", "foo") counter = q.get() db.run_in_transaction(decrement, counter.key(), amount=5) </code></pre> <p>I attempted to move my fetch of the category to before the transaction:</p> <pre><code>def add_item_txn(category_key, item_name): category = category_key.get() item_query = db.GqlQuery("SELECT * FROM Item WHERE name=:name AND ANCESTOR IS :category", name=item_name, category=category) item_results = item_query.fetch(1) if len(item_results) == 0: item = Item(parent=category, name=name) category_query = db.GqlQuery("SELECT * FROM Category WHERE name=:category_name", category_name="foo") category = category_query.get() if not category: category = Category(name=category_name, count=0) db.run_in_transaction(add_item_txn, category.key(), "bar") </code></pre> <p>This seemingly worked, but I found when I ran this with a number of requests that I had duplicate categories created, which makes sense, as the category is queried outside the transaction and multiple requests could create multiple categories.</p> <p>Does anyone have any idea how I can create these categories properly? I tried to put the category creation into a transaction, but received the error about ancestor queries only again.</p> <p>Thanks!</p> <p>Simon</p>
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    1. COAre you trying to avoid using key-names for Categories? The standard way to ensure uniqueness is by using db.get_or_insert(parent, key_name).
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    2. COYeah, that reason I'm avoiding it is because the category name can change, and if I change the name but have the key as the old name I'm afraid things will be very confusing...
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