I am using the code below to extract table names on a database at a GET call in a Flask app.:
session = db.session()
qry = session.query(models.BaseTableModel)
results = session.execute(qry)
table_names = []
for row in results:
for column, value in row.items():
#this seems like a bit of a hack
if column == "tables_table_name":
table_names.append(value)
print('{0}: '.format(table_names))
Given that tables in the database may added/deleted regularly, is the code above an efficient and reliable way to get the names of tables in a database?
One obvious optimization is to use row["tables_table_name"] instead of second loop.
Assuming that BaseTableModel is a table, which contains names of all other tables, than you're using the fastest approach to get this data.
Related
I want to return arrays with data from the entire row (so all columns), not just a single column. I can do this with a raw sql statement in Postgresql,
SELECT
array_agg(users.*)
FROM users
WHERE
l_name LIKE 'Br%'
GROUP BY f_name;
but when I try to do it with SqlAlchemy, I'm getting
sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) can't adapt type 'InstrumentedAttribute'
For example, when I execute this query, it works fine
query: Query[User] = session.query(array_agg(self.user.f_name))
But with this I get arrays of rows with only one column value in them (in this example, the first name of a user) whereas I want the entire row (all columns for a user).
I've tried explicitly listing multiple columns, but to no avail. For example I've tried this:
query: Query[User] = session.query(array_agg((self.user.f_name, self.user.l_name))))
But it doesn't work. I get the above error message.
You could use Python feature unpack for create
example = [func.array_agg(column) for column in self.example.__table__.columns]
query = self.dbsession.query(*attach)
And after join results
I want to specify the return values for a specific update in sqlalchemy.
The documentation of the underlying update statement (sqlalchemy.sql.expression.update) says it accepts a "returning" argument and the docs for the query object state that query.update() accepts a dictionary "update_args" which will be passed as the arguments to the query statement.
Therefore my code looks like this:
session.query(
ItemClass
).update(
{ItemClass.value: value_a},
synchronize_session='fetch',
update_args={
'returning': (ItemClass.id,)
}
)
However, this does not seem to work. It just returns the regular integer.
My question is now: Am I doing something wrong or is this simply not possible with a query object and I need to manually construct statements or write raw sql?
The full solution that worked for me was to use the SQLAlchemy table object directly.
You can get that table object and the columns from your model easily by doing
table = Model.__table__
columns = table.columns
Then with this table object, I can replicate what you did in the question:
from your_settings import db
update_statement = table.update().returning(table.id)\
.where(columns.column_name=value_one)\
.values(column_name='New column name')
result = db.session.execute(update_statement)
tuple_of_results = result.fetchall()
db.session.commit()
The tuple_of_results variable would contain a tuple of the results.
Note that you would have to run db.session.commit() in order to persist the changes to the database as you it is currently running within a transaction.
You could perform an update based on the current value of a column by doing something like:
update_statement = table.update().returning(table.id)\
.where(columns.column_name=value_one)\
.values(like_count=table_columns.like_count+1)
This would increment our numeric like_count column by one.
Hope this was helpful.
Here's a snippet from the SQLAlchemy documentation:
# UPDATE..RETURNING
result = table.update().returning(table.c.col1, table.c.col2).\
where(table.c.name=='foo').values(name='bar')
print result.fetchall()
I am doing something like this:
data = Model.where('something="something"')
random_data = data.rand(100..200)
returns:
NoMethodError (private method `rand' called for #<User::ActiveRecord_Relation:0x007fbab27d7ea8>):
Once I get this random data, I need to iterate through that data, like this:
random_data.each do |rd|
...
I know there's a way to fetch random data in MySQL, but I need to pick the random data like 400 times, so I think to load data once from database and 400 times to pick random number is more efficient than to run the query 400 times on MySQL.
But - how to get rid of that error?
NoMethodError (private method `rand' called for #<User::ActiveRecord_Relation:0x007fbab27d7ea8>):
Thank you in advance
I would add the following scope to the model (depends on the database you are using):
# to model/model.rb
# 'RANDOM' works with postgresql and sqlite, whereas mysql uses 'RAND'
scope :random, -> { order('RAND()') }
Then the following query would load a random number (in the range of 200-400) of objects in one query:
Model.random.limit(rand(200...400))
If you really want to do that in Rails and not in the database, then load all records and use sample:
Model.all.sample(rand(200..400))
But that to be slower (depending on the number of entries in the database), because Rails would load all records from the database and instantiate them what might take loads of memory.
It really depends how much effort you want to put into optimizing this, because there's more than one solution. Here's 2 options..
Something simple is to use ORDER BY RAND() LIMIT 400 to randomly select 400 items.
Alternatively, just select everything under the moon and then use Ruby to randomly pick 400 out of the total result set, ex:
data = Model.where(something: 'something').all # all is necessary to exec query
400.times do
data.sample # returns a random model
end
I wouldn't recommend the second method, but it should work.
Another way, which is not DB specific is :
def self.random_record
self.where('something = ? and id = ?', "something", rand(self.count))
end
The only things here is - 2 queries are being performed. self.count is doing one query - SELECT COUNT(*) FROM models and the other is your actual query to get a random record.
Well, now suppose you want n random records. Then write it like :
def self.random_records n
records = self.count
rand_ids = Array.new(n) { rand(records) }
self.where('something = ? and id IN (?)',
"something", rand_ids )
end
Use data.sample(rand(100..200))
for more info why rand is not working, read here https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4555
I just discovered that if you use the same name in bindparams twice in the same query, the second value clobbers the first one. For a contrived example:
db.session.query(MyTable).filter(
db.or_(
db.text("my_table.field = :value").bindparams(value=value1),
db.text("my_table.field = :value").bindparams(value=value2),
)
)
Here you would only get things with value2. value1 would not appear in the query.
Is there a general purpose way to fix this?
Btw, db.text() bits in my real query access nested jsonb properties, so please don't answer telling me to just use the column objects in this query in place of db.text().
I have a few rows of data pulled into business objects via linq-to-sql from large tables.
Now I want to get a few rows that don't match to test my comparison functions.
Using what I thought would work I get a NotSupportedException:
Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator.
Here's the code:
//This table has a 2 field primary key, the other has a single
var AllNonMatches = from c in dc.Acaps
where !Matches.Rows.Any((row) => row.Key.Key == c.AppId & row.Key.Value == c.SeqNbr)
select c;
foreach (var item in AllNonMatches.Take(100)) //Exception here
{}
The table has a compound primary key: AppId and SeqNbr.
The Matches.Rows is defined as a dictionary of keyvaluepair(appid,seqnbr).
and the local sequence it is referring to appears to be the local dictionary.
Could you provide more information on the structure and the name(s) of the table(s) plz?
Not sure what you're trying to do...
edit:
Ok.. I think I get it now...
It appears you can't merge/join local tables (dictionary) with a SQL table.
If you can, I'm afraid I don't know how to do it.
The simplest solution I can think of is to put those results in a table ("Match" for instance) with foreign keys related to your table "Acaps" and then use linq-to-sql, like:
var AllNonMatches = dc.Acaps.Where(p=>p.Matchs==null).Take(100).ToList();
Sorry I couldn't come up with any better =(
What about this:
var AllNonMatches = from c in dc.Acaps
where !(Matches.Rows.ContainsKey(c.AppId) && Matches.Rows.ContainsValue(c.SeqNbr))
select c;
That will work fine. I have also used a bitwise AND operator (&&) - I think thats the right term to help improve performance over the standard AND operator.