I am trying to understand a behaviour of Rails ActiveRecord with limit.
Let's say I have a model Car with only one attribute name.
Now in the console (rails console), if I type:
Car.limit(0).where(name: 'foo')
I get
SELECT "cars".* FROM "cars" WHERE "cars"."name" = 'foo' LIMIT 0
which is correct, but:
Car.limit(0).find_by_id(1)
gives
SELECT "cars".* FROM "cars" WHERE "cars"."id" = 1 LIMIT 1
Why is the limit option ignored?
Note that
Car.where(name:'foo').find_by_id(1)
gives
SELECT "cars".* FROM "cars" WHERE "cars"."name" = 'foo' AND "cars"."id" = 1 LIMIT 1
So find_by_id(or find) ignores limit but accepts where? How do you explain that?
Sidenote
I know that these queries don't make sense. I have an edge case in a project which executes Car.limit(0).find_by_id(1), the expected result was to get a nil object. I am trying to understand why find_by_id or find ignores limit.
find and find_by_ are methods used to retrieve a single instance. The result is either one object or nil if there are no objects to be found.
Because the method is supposed to return only one object it sets the limit to 1 regardless of what limit you specify.
If you want to specify a limit, use where as per your example.
Related
How is it possible to write a couchbase request in order to choose the first condition in where if it is defined or the second one if the first one is undefined.
I tried somethings like :
SELECT id FROM auto as a WHERE IFMISSINGORNULL(ARRAY_LENGTH(a.data[0])>1, ARRAY_LENGTH(a.data[1])>1)
In this request if a.data[0] is defined, I want to check the length of a.data[0] and if a.data[0] is not defined, I want to check the length of a.data[1].
Condition should look as follows
WHERE ARRAY_LENGTH(IFNULL(a.data[0], a.data[1])) > 1
IFNULL(a.data[0], a.data[1])
returns first argument if it is NOT NULL
second argument when first one is NULL
I am building a Rails app with the following models:
# vote.rb
class Vote < ApplicationRecord
belongs_to :person
belongs_to :show
scope :fulfilled, -> { where(fulfilled: true) }
scope :unfulfilled, -> { where(fulfilled: false) }
end
# person.rb
class Person < ApplicationRecord
has_many :votes, dependent: :destroy
def self.order_by_votes(show = nil)
count = 'nullif(votes.fulfilled, true)'
count = "case when votes.show_id = #{show.id} AND NOT votes.fulfilled then 1 else null end" if show
people = left_joins(:votes).group(:id).uniq!(:group)
people = people.select("people.*, COUNT(#{count}) AS people.vote_count")
people.order('people.vote_count DESC')
end
end
The idea behind order_by_votes is to sort People by the number of unfulfilled votes, either counting all votes, or counting only votes associated with a given Show.
This seem to work fine when I test against SQLite. But when I switch to Postgres I get this error:
Error:
PeopleControllerIndexTest#test_should_get_previously_on_show:
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR: column people.vote_count does not exist
LINE 1: ...s"."show_id" = $1 GROUP BY "people"."id" ORDER BY people.vot...
^
If I dump the SQL using #people.to_sql, this is what I get:
SELECT people.*, COUNT(nullif(votes.fulfilled, true)) AS people.vote_count FROM "people" LEFT OUTER JOIN "votes" ON "votes"."person_id" = "people"."id" GROUP BY "people"."id" ORDER BY people.vote_count DESC
Why is this failing on Postgres but working on SQLite? And what should I be doing instead to make it work on Postgres?
(PS: I named the field people.vote_count, with a dot, so I can access it in my view without having to do another SQL query to actually view the vote count for each person in the view (not sure if this works) but I get the same error even if I name the field simply vote_count.)
(PS2: I recently added the .uniq!(:group) because of some deprecation warning for Rails 6.2, but I couldn't find any documentation for it so I am not sure I am doing it right, still the error is there without that part.)
Are you sure you're not getting a syntax error from PostgreSQL somewhere? If you do something like this:
select count(*) as t.vote_count from t ... order by t.vote_count
I get a syntax error before PostgreSQL gets to complain about there being no t.vote_count column.
No matter, the solution is to not try to put your vote_count in the people table:
people = people.select("people.*, COUNT(#{count}) AS vote_count")
...
people.order(vote_count: :desc)
You don't need it there, you'll still be able to reference the vote_count just like any "normal" column in people. Anything in the select list will appear as an accessor in the resultant model instances whether they're columns or not, they won't show up in the #inspect output (since that's generated based on the table's columns) but you call the accessor methods nonetheless.
Historically there have been quite a few AR problems (and bugs) in getting the right count by just using count on a scope, and I am not sure they are actually all gone.
That depends on the scope (AR version, relations, group, sort, uniq, etc). A defaut count call that a gem has to generically use on a scope is not a one-fit-all solution. For that known reason Pagy allows you to pass the right count to its pagy method as explained in the Pagy documentation.
Your scope might become complex and the default pagy collection.count(:all) may not get the actual count. In that case you can get the right count with some custom statement, and pass it to pagy.
#pagy, #records = pagy(collection, count: your_count)
Notice: pagy will efficiently skip its internal count query and will just use the passed :count variable.
So... just get your own calculated count and pass it to pagy, and it will not even try to use the default.
EDIT: I forgot to mention: you may want to try the pagy arel extra that:
adds specialized pagination for collections from sql databases with GROUP BY clauses, by computing the total number of results with COUNT(*) OVER ().
Thanks to all the comments and answers I have finally found a solution which I think is the best way to solve this.
First of, the issue occurred when I called pagy which tried to count my scope by appending .count(:all). This is what caused the errors. The solution was to not create a "field" in select() and use it in .order().
So here is the proper code:
def self.order_by_votes(show = nil)
count = if show
"case when votes.show_id = #{show.id} AND NOT votes.fulfilled then 1 else null end"
else
'nullif(votes.fulfilled, true)'
end
left_joins(:votes).group(:id)
.uniq!(:group)
.select("people.*, COUNT(#{count}) as vote_count")
.order(Arel.sql("COUNT(#{count}) DESC"))
end
This sorts the number of people on the number of unfulfilled votes for them, with the ability to count only votes for a given show, and it works with pagy(), and pagy_arel() which in my case is a much better fit, so the results can be properly paginated.
I am new to rails. What I see that there are a lot of ways to find a record:
find_by_<columnname>(<columnvalue>)
find(:first, :conditions => { <columnname> => <columnvalue> }
where(<columnname> => <columnvalue>).first
And it looks like all of them end up generating exactly the same SQL. Also, I believe the same is true for finding multiple records:
find_all_by_<columnname>(<columnvalue>)
find(:all, :conditions => { <columnname> => <columnvalue> }
where(<columnname> => <columnvalue>)
Is there a rule of thumb or recommendation on which one to use?
where returns ActiveRecord::Relation
Now take a look at find_by implementation:
def find_by
where(*args).take
end
As you can see find_by is the same as where but it returns only one record. This method should be used for getting 1 record and where should be used for getting all records with some conditions.
Edit:
This answer is very old and other, better answers have come up since this post was made. I'd advise looking at the one posted below by #Hossam Khamis for more details.
Use whichever one you feel suits your needs best.
The find method is usually used to retrieve a row by ID:
Model.find(1)
It's worth noting that find will throw an exception if the item is not found by the attribute that you supply. Use where (as described below, which will return an empty array if the attribute is not found) to avoid an exception being thrown.
Other uses of find are usually replaced with things like this:
Model.all
Model.first
find_by is used as a helper when you're searching for information within a column, and it maps to such with naming conventions. For instance, if you have a column named name in your database, you'd use the following syntax:
Model.find_by(name: "Bob")
.where is more of a catch all that lets you use a bit more complex logic for when the conventional helpers won't do, and it returns an array of items that match your conditions (or an empty array otherwise).
Model.find
1- Parameter: ID of the object to find.
2- If found: It returns the object (One object only).
3- If not found: raises an ActiveRecord::RecordNotFound exception.
Model.find_by
1- Parameter: key/value
Example:
User.find_by name: 'John', email: 'john#doe.com'
2- If found: It returns the object.
3- If not found: returns nil.
Note: If you want it to raise ActiveRecord::RecordNotFound use find_by!
Model.where
1- Parameter: same as find_by
2- If found: It returns ActiveRecord::Relation containing one or more records matching the parameters.
3- If not found: It return an Empty ActiveRecord::Relation.
There is a difference between find and find_by in that find will return an error if not found, whereas find_by will return null.
Sometimes it is easier to read if you have a method like find_by email: "haha", as opposed to .where(email: some_params).first.
Since Rails 4 you can do:
User.find_by(name: 'Bob')
which is the equivalent find_by_name in Rails 3.
Use #where when #find and #find_by are not enough.
The accepted answer generally covers it all, but I'd like to add something,
just incase you are planning to work with the model in a way like updating, and you are retrieving a single record(whose id you do not know), Then find_by is the way to go, because it retrieves the record and does not put it in an array
irb(main):037:0> #kit = Kit.find_by(number: "3456")
Kit Load (0.9ms) SELECT "kits".* FROM "kits" WHERE "kits"."number" =
'3456' LIMIT 1
=> #<Kit id: 1, number: "3456", created_at: "2015-05-12 06:10:56",
updated_at: "2015-05-12 06:10:56", job_id: nil>
irb(main):038:0> #kit.update(job_id: 2)
(0.2ms) BEGIN Kit Exists (0.4ms) SELECT 1 AS one FROM "kits" WHERE
("kits"."number" = '3456' AND "kits"."id" != 1) LIMIT 1 SQL (0.5ms)
UPDATE "kits" SET "job_id" = $1, "updated_at" = $2 WHERE "kits"."id" =
1 [["job_id", 2], ["updated_at", Tue, 12 May 2015 07:16:58 UTC +00:00]]
(0.6ms) COMMIT => true
but if you use where then you can not update it directly
irb(main):039:0> #kit = Kit.where(number: "3456")
Kit Load (1.2ms) SELECT "kits".* FROM "kits" WHERE "kits"."number" =
'3456' => #<ActiveRecord::Relation [#<Kit id: 1, number: "3456",
created_at: "2015-05-12 06:10:56", updated_at: "2015-05-12 07:16:58",
job_id: 2>]>
irb(main):040:0> #kit.update(job_id: 3)
ArgumentError: wrong number of arguments (1 for 2)
in such a case you would have to specify it like this
irb(main):043:0> #kit[0].update(job_id: 3)
(0.2ms) BEGIN Kit Exists (0.6ms) SELECT 1 AS one FROM "kits" WHERE
("kits"."number" = '3456' AND "kits"."id" != 1) LIMIT 1 SQL (0.6ms)
UPDATE "kits" SET "job_id" = $1, "updated_at" = $2 WHERE "kits"."id" = 1
[["job_id", 3], ["updated_at", Tue, 12 May 2015 07:28:04 UTC +00:00]]
(0.5ms) COMMIT => true
Apart from accepted answer, following is also valid
Model.find() can accept array of ids, and will return all records which matches.
Model.find_by_id(123) also accept array but will only process first id value present in array
Model.find([1,2,3])
Model.find_by_id([1,2,3])
The answers given so far are all OK.
However, one interesting difference is that Model.find searches by id; if found, it returns a Model object (just a single record) but throws an ActiveRecord::RecordNotFound otherwise.
Model.find_by is very similar to Model.find and lets you search any column or group of columns in your database but it returns nil if no record matches the search.
Model.where on the other hand returns a Model::ActiveRecord_Relation object which is just like an array containing all the records that match the search. If no record was found, it returns an empty Model::ActiveRecord_Relation object.
I hope these would help you in deciding which to use at any point in time.
Suppose I have a model User
User.find(id)
Returns a row where primary key = id. The return type will be User object.
User.find_by(email:"abc#xyz.com")
Returns first row with matching attribute or email in this case. Return type will be User object again.
Note :- User.find_by(email: "abc#xyz.com") is similar to User.find_by_email("abc#xyz.com")
User.where(project_id:1)
Returns all users in users table where attribute matches.
Here return type will be ActiveRecord::Relation object. ActiveRecord::Relation class includes Ruby's Enumerable module so you can use it's object like an array and traverse on it.
Both #2s in your lists are being deprecated. You can still use find(params[:id]) though.
Generally, where() works in most situations.
Here's a great post: https://web.archive.org/web/20150206131559/http://m.onkey.org/active-record-query-interface
The best part of working with any open source technology is that you can inspect length and breadth of it.
Checkout this link
find_by ~> Finds the first record matching the specified conditions. There is no implied ordering so if order matters, you should specify it yourself. If no record is found, returns nil.
find ~> Finds the first record matching the specified conditions , but if no record is found, it raises an exception but that is done deliberately.
Do checkout the above link, it has all the explanation and use cases for the following two functions.
I will personally recommend using
where(< columnname> => < columnvalue>)
I want to retrieve all the files from a cabinet (called 'Wombat Insurance Co'). Currently I am using this DQL query:
select r_object_id, object_name from dm_document(all)
where folder('/Wombat Insurance Co', descend);
This is ok except it only returns a maximum of 100 results. If there are 5000 files in the cabinet I want to get all 5000 results. Is there a way to use pagination to get all the results?
I have tried this query:
select r_object_id, object_name from dm_document(all)
where folder('/Wombat Insurance Co', descend)
ENABLE (RETURN_RANGE 0 100 'r_object_id DESC');
with the intention of getting results in 100 file increments, but this query gives me an error when I try to execute it. The error says this:
com.emc.documentum.fs.services.core.CoreServiceException: "QUERY" action failed.
java.lang.Exception: [DM_QUERY2_E_UNRECOGNIZED_HINT]error:
"RETURN_RANGE is an unknown hint or is being used incorrectly."
I think I am using the RETURN_RANGE hint correctly, but maybe I'm not. Any help would be appreciated!
I have also tried using the hint ENABLE(FETCH_ALL_RESULTS 0) but this still only returns a maximum of 100 results.
To clarify, my question is: how can I get all the files from a cabinet?
You have already accepted an answer which is using DFS.
Since your are playing with DFC, these information might help you.
DFS:
If you are using DFS, you have to aware about the number of concurrent sessions that you can consume with DFS.
I think it is 100 or 150.
DFC:
Actually there is a limit that you can fetch via DFC (I'm not sure with DFS).
Go to your DFC application(webtop or da or anything) and check the dfc.properties file.
# Maximum number of results to retrieve by a query search.
# min value: 1, max value: 10000000
#
dfc.search.max_results = 100
# Maximum number of results to retrieve per source by a query search.
# min value: 1, max value: 10000000
#
dfc.search.max_results_per_source = 400
dfc.properties.full or similar file is there and you can verify these values according to your system.
And I'm talking about the ContentServer side, not the client side dfc.properties file.
If you use ENABLE (RETURN_TOP) hint with DFC, there are 2 ways to fetch the results from the ContentServer.
Object based
Row based
You have to configure this by using the parameter return_top_results_row_based in the server.ini file.
All of these changes for the documentum server side, not for your DFC/DQL client.
Aha, I've figured it out. Using DFS with Java (an abstraction layer on top of DFC) you can set the starting index for query results:
String queryStr = "select r_object_id, object_name from dm_document(all)
where folder('/Wombat Insurance Co', descend);"
PassthroughQuery query = new PassthroughQuery();
query.setQueryString(queryStr);
query.addRepository(repositoryStr);
QueryExecution queryEx = new QueryExecution();
queryEx.setCacheStrategyType(CacheStrategyType.DEFAULT_CACHE_STRATEGY);
queryEx.setStartingIndex(currentIndex); // set start index here
OperationOptions operationOptions = null;
// will return 100 results starting from currentIndex
QueryResult queryResult = queryService.execute(query, queryEx, operationOptions);
You can just increment the currentIndex variable to get all results.
Well, the hint is being used incorrectly. Start with 1, not 0.
There is no built-in limit in DQL itself. All results are returned by default. The reason you get only 100 results must have something to do with the way you're using DFC (or whichever other client you are using). Using IDfCollection in the following way will surely return everything:
IDfQuery query = new DfQuery("SELECT r_object_id, object_name "
+ "FROM dm_document(all) WHERE FOLDER('/System', DESCEND)");
IDfCollection coll = query.execute(session, IDfQuery.DF_READ_QUERY);
int i = 0;
while (coll.next()) i++;
System.out.println("Number of results: " + i);
In a test environment (CS 6.7 SP1 x64, MS SQL), this outputs:
Number of results: 37162
Now, there's proof. Using paging is however a good idea if you want to improve the overall performance in your application. As mentioned, start counting with the number 1:
ENABLE(RETURN_RANGE 1 100 'r_object_id DESC')
This way of paging requires that sorting be specified in the hint rather than as a DQL statement. If all you want is the first 100 records, try this hint instead:
ENABLE(RETURN_TOP 100)
In this case sorting with ORDER BY will work as you'd expect.
Lastly, note that adding (all) will not only find all documents matching the specified qualification, but all versions of every document. If this was your intention, that's fine.
I've worked with DFC API (with Java) for a while but I don't remember any default limit on queries, IIRC we've always got all of the documents, there weren't any limit. Actually (according to my notes) we have to set the limit explicitly with, for example, enable (return_top 2000). (As far I know the syntax might be depend on the DBMS behind EMC Documentum.)
Just a guess: check your dfc.properties file.
I'm trying to select the User Id as a variable in the console however I keep ending up with:
[#<User id: 4>]
The find statement I have tried is:
userid = User.select('id').where('username = ?', 'uwZgf')
I've also tried with find_by_sql with same result.
What do I need to get the value out instead of the hash?
What you've got there is an array of one User object.
User.select(...).where(...).first.id
Would do the trick (you'd probably want to check the value returned by first before trying to call id on it.
You might find
User.find_by_username('foo').try(:id)
more readable.