I am attempt to create a constraint like this:
CREATE CONSTRAINT ON (a:forecast) ASSERT a.match IS UNIQUE
but I get this error:
Unable to create Constraint( name='constraint_9615361a',
type='UNIQUENESS', schema=(:forecast {match}) ):
I have the Neo4J community edition 4.2.3, but judging by the documentation I should be allowed to create this type of constraint. What gives?
You can check if there is uniqueness on forecast {match} by running this.
MATCH (a:forecast)
RETURN a.match, count(*) LIMIT 5
If you see a value > 1 in count() then forecast.match is not unique.
Note: count() is sorting in descending order in my desktop. In case it is not, then run below:
MATCH (a:forecast)
WITH a.match as match, count(*) as cnt
RETURN match, cnt ORDER by cnt DESC LIMIT 5
Late reply, but still.
I was struck by the same error now (with 4.4.3) and it was lack of free space on drive.
Wasted half an hour, digging in absolutely wrong direction.
I create database with --skip-duplicate-nodes=true flag, so of course there couldn't be any duplicates.
Related
I have to perform the following SQL query on a Solr collection:
select c.CONTRACT
from contracts c
where c.LIMIT_TYPE = 4
and c.TRANSACTION_DATE_DB = (select max(TRANSACTION_DATE_DB) from contracts c2 where c.CONTRACT = c2.CONTRACT and c2.LIMIT_TYPE = 4)
and c.STATUS = 'ENABLED'
group by c.CONTRACT
order by c.CONTRACT desc;
The objective of the query is the following:
the subquery find, for each contract, the record with last transaction date for limit type 4
the upper query checks if, for each contract, if the record returned has status enabled.
I've seen that there are two possible ways to perform this query using solr: join and subquery
Here below I'll paste my 100th attempt using subquery:
http://localhost:8983/solr/contracts_collection/select?
q=*:*
&indent=true
fq=LIMIT_TYPE:4&
fq=STATUS:ENABLED
&fl=*,mySub:[subquery]
&mySub.q=*
&mySub.fq={!term f=CONTRACT_IDENTITY v=$row.CONTRACT_IDENTITY}
&mySub.fq=LIMIT_TYPE:4
&mySub.fl=CONTRACT_IDENTITY,TRANSACTION_DATE_DB,LIMIT_TYPE
&mySub.sort=TRANSACTION_DATE_DB+desc
&mySub.rows=1
The subquery is correct. However, for each contract, the result contains all the records for that contract. In fact, I'm missing here how to make the upper query return, for each contract, only the record having the same transaction_date_db returned by the subquery.
I tried to add the following condition:
&mySub.fq={!term f=TRANSACTION_DATE_DB v=$row.TRANSACTION_DATE_DB}
But still, I don't have the desired result.
I've also saw join, but I've failed miserably :P
Does anyone have any suggestion? Going crazy for two days now.
Thanks a lot
I am getting this error in the pg production mode, but its working fine in sqlite3 development mode.
ActiveRecord::StatementInvalid in ManagementController#index
PG::Error: ERROR: column "estates.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = ...
^
: SELECT "estates".* FROM "estates" WHERE "estates"."Mgmt" = 'Mazzey' GROUP BY user_id
#myestate = Estate.where(:Mgmt => current_user.Company).group(:user_id).all
If user_id is the PRIMARY KEY then you need to upgrade PostgreSQL; newer versions will correctly handle grouping by the primary key.
If user_id is neither unique nor the primary key for the 'estates' relation in question, then this query doesn't make much sense, since PostgreSQL has no way to know which value to return for each column of estates where multiple rows share the same user_id. You must use an aggregate function that expresses what you want, like min, max, avg, string_agg, array_agg, etc or add the column(s) of interest to the GROUP BY.
Alternately you can rephrase the query to use DISTINCT ON and an ORDER BY if you really do want to pick a somewhat arbitrary row, though I really doubt it's possible to express that via ActiveRecord.
Some databases - including SQLite and MySQL - will just pick an arbitrary row. This is considered incorrect and unsafe by the PostgreSQL team, so PostgreSQL follows the SQL standard and considers such queries to be errors.
If you have:
col1 col2
fred 42
bob 9
fred 44
fred 99
and you do:
SELECT col1, col2 FROM mytable GROUP BY col1;
then it's obvious that you should get the row:
bob 9
but what about the result for fred? There is no single correct answer to pick, so the database will refuse to execute such unsafe queries. If you wanted the greatest col2 for any col1 you'd use the max aggregate:
SELECT col1, max(col2) AS max_col2 FROM mytable GROUP BY col1;
I recently moved from MySQL to PostgreSQL and encountered the same issue. Just for reference, the best approach I've found is to use DISTINCT ON as suggested in this SO answer:
Elegant PostgreSQL Group by for Ruby on Rails / ActiveRecord
This will let you get one record for each unique value in your chosen column that matches the other query conditions:
MyModel.where(:some_col => value).select("DISTINCT ON (unique_col) *")
I prefer DISTINCT ON because I can still get all the other column values in the row. DISTINCT alone will only return the value of that specific column.
After often receiving the error myself I realised that Rails (I am using rails 4) automatically adds an 'order by id' at the end of your grouping query. This often results in the error above. So make sure you append your own .order(:group_by_column) at the end of your Rails query. Hence you will have something like this:
#problems = Problem.select('problems.username, sum(problems.weight) as weight_sum').group('problems.username').order('problems.username')
#myestate1 = Estate.where(:Mgmt => current_user.Company)
#myestate = #myestate1.select("DISTINCT(user_id)")
this is what I did.
In SQLite (development) I don't have any errors, but in production with Postgres I get the following error. I don't really understand the error.
PG::Error: ERROR: column "commits.updated_at" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: ...mmits"."user_id" = 1 GROUP BY mission_id ORDER BY updated_at...
^
: SELECT COUNT(*) AS count_all, mission_id AS mission_id FROM "commits" WHERE "commits"."user_id" = 1 GROUP BY mission_id ORDER BY updated_at DESC
My controller method:
def show
#user = User.find(params[:id])
#commits = #user.commits.order("updated_at DESC").page(params[:page]).per(25)
#missions_commits = #commits.group("mission_id").count.length
end
UPDATE:
So i digged further into this PostgreSQL specific annoyance and I am surprised that this exception is not mentioned in the Ruby on Rails Guide.
I am using psql (PostgreSQL) 9.1.11
So from what I understand, I need to specify which column that should be used whenever you use the GROUP_BY clause. I thought using SELECT would help, which can be annoying if you need to SELECT a lot of columns.
Interesting discussion here
Anyways, when I look at the error, everytime the cursor is pointed to updated_at. In the SQL query, rails will always ORDER BY updated_at. So I have tried this horrible query:
#commits.group("mission_id, date(updated_at)")
.select("date(updated_at), count(mission_id)")
.having("count(mission_id) > 0")
.order("count(mission_id)").length
which gives me the following SQL
SELECT date(updated_at), count(mission_id)
FROM "commits"
WHERE "commits"."user_id" = 1
GROUP BY mission_id, date(updated_at)
HAVING count(mission_id) > 0
ORDER BY updated_at DESC, count(mission_id)
LIMIT 25 OFFSET 0
the error is the same.
Note that no matter what it will ORDER BY updated_at, even if I wanted to order by something else.
Also I don't want to group the records by updated_at just by mission_id.
This PostgreSQL error is just misleading and has little explanation to solving it. I have tried many formulas from the stackoverflow sidebar, nothing works and always the same error.
UPDATE 2:
So I got it to work, but it needs to group the updated_at because of the automatic ORDER BY updated_at. How do I count only by mission_id?
#missions_commits = #commits.group("mission_id, updated_at").count("mission_id").size
I guest you want to show general number of distinct Missions related with Commits, anyway it won't be number on page.
Try this:
#commits = #user.commits.order("updated_at DESC").page(params[:page]).per(25)
#missions_commits = #user.commits.distinct.count(:mission_id)
However if you want to get the number of distinct Missions on page I suppose it should be:
#missions_commits = #commits.collect(&:mission_id).uniq.count
Update
In Rails 3, distinct did not exist, but pure SQL counting should be used this way:
#missions_commits = #user.commits.count(:mission_id, distinct: true)
See the docs for PostgreSQL GROUP BY here:
http://www.postgresql.org/docs/9.3/interactive/sql-select.html#SQL-GROUPBY
Basically, unlike Sqlite (and MySQL) postgres requires that any columns selected or ordered on must appear in an aggregate function or the group by clause.
If you think it through, you'll see that this actually makes sense. Sqlite/MySQL cheat under the hood and silently drop those fields (not sure that's technically what happens).
Or thinking about it another way if you are grouping by a field, what's the point of ordering it? How would that even make sense unless you also had an aggregate function on the ordered field?
Im trying to query my db for records that are similar to the currently viewed record (based on taggings), which I have working but I would like to randomize the order.
my development environment is mysql so I would do something like:
#tattoos = Tattoo.tagged_with(tags, :any => true).order("RAND()").limit(6)
which works, but my production environment is heroku which is using postgresql so I tried using this:
#tattoos = Tattoo.tagged_with(tags, :any => true).order("RANDOM()").limit(6)
but I get the following error:
ActionView::Template::Error (PGError: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
SELECT DISTINCT tattoos.* FROM "tattoos" JOIN taggings
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477 ON
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.taggable_id = tattoos.id AND
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.taggable_type = 'Tattoo' WHERE
(tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.tag_id = 3 OR
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.tag_id = 4 OR
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.tag_id = 5 OR
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.tag_id = 24 OR
tattoos_taggings_color_fantasy_newschool_nerdy_tv_477.tag_id = 205) ORDER BY RANDOM() LIMIT 6):
After analyzing the query more closely, I have to correct my first draft. The query would require a DISTINCT or GROUP BY the way it is.
The (possibly) duplicate tattoos.* come from first joining to (possibly) multiple rows in the table taggings. Your query engine then tries to get rid of such duplicates again by using DISTINCT - in a syntactically illegal way.
DISTINCT basically sorts the resulting rows by the resulting columns from left to right and picks the first for each set of duplicates. That's why the leftmost ORDER BY column have to match the SELECT list.
MySQL is more permissive and allows the non-standard use of DISTINCT, but PostgreSQL throws an error.
ORMs often produce ineffective SQL statements (they are just crutches after all). However, if you use appropriate PostgreSQL libraries, such an illegal statement shouldn't be produced to begin with. I am no Ruby expert, but something's fishy here.
The query is also very ugly and inefficient.
There are several ways to fix it. For instance:
SELECT *
FROM (<query without ORDER BY and LIMIT>) x
ORDER BY RANDOM()
LIMIT 6
Or, better yet, rewrite the query with this faster, cleaner alternative doing the same:
SELECT ta.*
FROM tattoos ta
WHERE EXISTS (
SELECT 1
FROM taggings t
WHERE t.taggable_id = ta .id
AND t.taggable_type = 'Tattoo'
AND t.tag_id IN (3, 4, 5, 24, 205)
)
ORDER BY RANDOM()
LIMIT 6;
You'll have to implement it in Ruby yourself.
not sure about the random, as it should work.
But take a note of http://railsforum.com/viewtopic.php?id=36581
which has code that might suit you
/lib/agnostic_random.rb
module AgnosticRandom
def random
case DB_ADAPTER
when "mysql" then "RAND()"
when "postgresql" then "RANDOM()"
end
end
end
/initializers/extend_ar.rb (name doesn't matter)
ActiveRecord::Base.extend AgnosticRandom
In my Rails app I've run into an issue a couple times that I'd like to know how other people solve:
I have certain records where a value is optional, so some records have a value and some are null for that column.
If I order by that column on some databases the nulls sort first and on some databases the nulls sort last.
For instance, I have Photos which may or may not belong to a Collection, ie there are some Photos where collection_id=nil and some where collection_id=1 etc.
If I do Photo.order('collection_id desc) then on SQLite I get the nulls last but on PostgreSQL I get the nulls first.
Is there a nice, standard Rails way to handle this and get consistent performance across any database?
I'm no expert at SQL, but why not just sort by if something is null first then sort by how you wanted to sort it.
Photo.order('collection_id IS NULL, collection_id DESC') # Null's last
Photo.order('collection_id IS NOT NULL, collection_id DESC') # Null's first
If you are only using PostgreSQL, you can also do this
Photo.order('collection_id DESC NULLS LAST') #Null's Last
Photo.order('collection_id DESC NULLS FIRST') #Null's First
If you want something universal (like you're using the same query across several databases, you can use (courtesy of #philT)
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
Even though it's 2017 now, there is still yet to be a consensus on whether NULLs should take precedence. Without you being explicit about it, your results are going to vary depending on the DBMS.
The standard doesn't specify how NULLs should be ordered in comparison with non-NULL values, except that any two NULLs are to be considered equally ordered, and that NULLs should sort either above or below all non-NULL values.
source, comparison of most DBMSs
To illustrate the problem, I compiled a list of a few most popular cases when it comes to Rails development:
PostgreSQL
NULLs have the highest value.
By default, null values sort as if larger than any non-null value.
source: PostgreSQL documentation
MySQL
NULLs have the lowest value.
When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC.
source: MySQL documentation
SQLite
NULLs have the lowest value.
A row with a NULL value is higher than rows with regular values in ascending order, and it is reversed for descending order.
source
Solution
Unfortunately, Rails itself doesn't provide a solution for it yet.
PostgreSQL specific
For PostgreSQL you could quite intuitively use:
Photo.order('collection_id DESC NULLS LAST') # NULLs come last
MySQL specific
For MySQL, you could put the minus sign upfront, yet this feature seems to be undocumented. Appears to work not only with numerical values, but with dates as well.
Photo.order('-collection_id DESC') # NULLs come last
PostgreSQL and MySQL specific
To cover both of them, this appears to work:
Photo.order('collection_id IS NULL, collection_id DESC') # NULLs come last
Still, this one does not work in SQLite.
Universal solution
To provide cross-support for all DBMSs you'd have to write a query using CASE, already suggested by #PhilIT:
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
which translates to first sorting each of the records first by CASE results (by default ascending order, which means NULL values will be the last ones), second by calculation_id.
Photo.order('collection_id DESC NULLS LAST')
I know this is an old one but I just found this snippet and it works for me.
Put minus sign in front of column_name and reverse the order direction. It works on mysql. More details
Product.order('something_date ASC') # NULLS came first
Product.order('-something_date DESC') # NULLS came last
Bit late to the show but there is a generic SQL way to do it. As usual, CASE to the rescue.
Photo.order('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id')
The easiest way is to use:
.order('name nulls first')
For posterity's sake, I wanted to highlight an ActiveRecord error relating to NULLS FIRST.
If you try to call:
Model.scope_with_nulls_first.last
Rails will attempt to call reverse_order.first, and reverse_order is not compatible with NULLS LAST, as it tries to generate the invalid SQL:
PG::SyntaxError: ERROR: syntax error at or near "DESC"
LINE 1: ...dents" ORDER BY table_column DESC NULLS LAST DESC LIMIT...
This was referenced a few years ago in some still-open Rails issues (one, two, three). I was able to work around it by doing the following:
scope :nulls_first, -> { order("table_column IS NOT NULL") }
scope :meaningfully_ordered, -> { nulls_first.order("table_column ASC") }
It appears that by chaining the two orders together, valid SQL gets generated:
Model Load (12.0ms) SELECT "models".* FROM "models" ORDER BY table_column IS NULL DESC, table_column ASC LIMIT 1
The only downside is that this chaining has to be done for each scope.
Rails 6.1 adds nulls_first and nulls_last methods to Arel for PostgreSQL.
Example:
User.order(User.arel_table[:login_count].desc.nulls_last)
Source: https://www.bigbinary.com/blog/rails-6-1-adds-nulls-first-and-nulls-last-to-arel
Here are some Rails 6 solutions.
The answer by #Adam Sibik is a great summary about the difference between various database systems.
Unfortunately, though, some of the presented solutions, including "Universal solution" and "PostgreSQL and MySQL specific", would not work any more with Rails 6 (ActiveRecord 6) as a result of its changed specification of order() not accepting some raw SQLs (I confirm the "PostgreSQL specific" solution still works as of Rails 6.1.4). For the background of this change, see, for example,
"Updates for SQL Injection in Rails 6.1" by Justin.
To circumvent the problem, you can wrap around the SQL statements with Arel.sql as follows, where NULLs come last, providing you are 100% sure the SQL statements you give are safe.
Photo.order(Arel.sql('CASE WHEN collection_id IS NULL THEN 1 ELSE 0 END, collection_id'))
Just for reference, if you want to sort by a Boolean column (is_ok, as an example) in the order of [TRUE, FALSE, NULL] regardless of the database systems, either of these should work:
Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 ELSE 0 END, is_ok DESC'))
Photo.order(Arel.sql('CASE WHEN is_ok IS NULL THEN 1 WHEN is_ok IS TRUE THEN -1 ELSE 0 END'))
(n.b., SQLite does not have the Boolean type and so the former may be safer arguably, though it should not matter because Rails should guarantee the value is either 0 or 1 (or NULL).)
In my case I needed sort lines by start and end date by ASC, but in few cases end_date was null and that lines should be in above, I used
#invoice.invoice_lines.order('start_date ASC, end_date ASC NULLS FIRST')
Adding arrays together will preserve order:
#nonull = Photo.where("collection_id is not null").order("collection_id desc")
#yesnull = Photo.where("collection_id is null")
#wanted = #nonull+#yesnull
http://www.ruby-doc.org/core/classes/Array.html#M000271
It seems like you'd have to do it in Ruby if you want consistent results across database types, as the database itself interprets whether or not the NULLS go at the front or end of the list.
Photo.all.sort {|a, b| a.collection_id.to_i <=> b.collection_id.to_i}
But that is not very efficient.