[mongoid] Using identity map values but disabling adding new identity map values

On Sunday, 6 January, 2013 at 6:56 PM, Jon wrote:

We use #only in our app, and per https://groups.google.com/forum/?fromgroups=#!searchin/mongoid/only$20identity$20map/mongoid/QsyYKiA0qYI/uZrOokIsX2EJ we wrap all #only queries in a unit of work like this:

Mongoid.unit_of_work(:disable => :current) do
user = User.only(:email).find(user_id)
end

However, this disables reading from the identity map as well as writing to the identity map. That is, this makes two database calls:

# Loads User into the identity map
user = User.find(user_id)
# Ignores the identity map, so makes another database call
Mongoid.unit_of_work(:disable => :current) do
user = User.only(:email).find(user_id)
end


Is there a way to disable writing to the identity map so #only queries don't go into it, but enable the use of the identity map for #only queries? We have a pretty large application and I'm pretty sure that given this behavior, we're making unnecessary database calls somewhere. This is fairly easy to do, too: One class might operate on a User object whereas another class operates on some object that belongs_to User that only needs access to certain fields so it uses #only when it gets the User.

On Sunday, January 6, 2013 8:03:29 PM UTC-5, Arthur Neves wrote:

I was just reading the other thread about .only and identity map, and I
guess this is a bug that should be fixed. I am saying that because having
to disable identity map for every .only query is really painful, i guess.
Just taking active record as example<https://github.com/rails/rails/blob/3-2-stable/activerecord/lib/active_record/identity_map.rb#L93> ,
they disable add method when all the attributes are not there. So in my
perspective, this should be implement it as well in mongoid.

Just to answer your question, there is no way to disable identity map per
Document/Model. the only way to disable is using unit_of_work . If you want
to have another behaviour you could extend IdentityMap and implement what
ever you need, also you would have to implement the UnitOfWork module to
use your identity map instead of the main one.

--
Arthur Neves
*@arthurnn <http://twitter.com/arthurnn>*

On Sunday, 6 January, 2013 at 6:56 PM, Jon wrote:

We use #only in our app, and per
https://groups.google.com/forum/?fromgroups=#!searchin/mongoid/only$20identity$20map/mongoid/QsyYKiA0qYI/uZrOokIsX2EJwe wrap all #only queries in a unit of work like this:

Mongoid.unit_of_work(:disable => :current) do
user = User.only(:email).find(user_id)
end

However, this disables reading from the identity map as well as writing to
the identity map. That is, this makes two database calls:

# Loads User into the identity map
user = User.find(user_id)
# Ignores the identity map, so makes another database call
Mongoid.unit_of_work(:disable => :current) do
user = User.only(:email).find(user_id)
end

Is there a way to disable writing to the identity map so #only queries
don't go into it, but enable the use of the identity map for #only queries?
We have a pretty large application and I'm pretty sure that given this
behavior, we're making unnecessary database calls somewhere. This is fairly
easy to do, too: One class might operate on a User object whereas another
class operates on some object that belongs_to User that only needs access
to certain fields so it uses #only when it gets the User.

On Tuesday, 8 January, 2013 at 11:21 AM, Jon wrote:

Durran, what do you think the right way to fix this should be? I see two paths:

1. Documents retrieved via #only do not get added to the IdentityMap. This way no one has to remember to wrap all #only calls in a unit_of_work.
2. Create a new unit_of_work option that lets you read from the identity map but not add any documents to it.

(1) seems like the right call to me since if we agree in both principle and in practice that partial documents should never be added to the IdentityMap, the ODM should enforce it instead of requiring developers to in their code.

Thoughts?