Currently the only way I know to sub select in rails is with arel,
for exmaple -
sub = x.where(y:'x').project(:id)
select = a.where(a[:x_id].in(sub))
question is,
if x is using the acts as taggable on gem and need to filtered by a specific tag, use with tagged_with method.
How can I still achive same database efficiency, it looks like the tagged with method override the projection.
thanks,
You don't need Arel to build sub selects in Rails:
sub = X.where(y: 'x')
select = A.where(x_id: sub)
generates the following SQL, assuming A's table name is as and X's is xs:
SELECT "as".* FROM "as" WHERE "as"."x_id" IN (SELECT "xs"."id" FROM "xs" WHERE "xs"."y" = 'x')
Testing with tagged_with worked: A.where(x_id: X.tagged_with('my_tag')) generates the expected SQL, at least for Rails 5.1, version on which I've tested.
Edit
You can specify the column used inside the subselect if needed. If you don't specify it, the primary key column is the default:
sub = X.where(y: 'x').select(:x_y_id)
select = A.where(x_id: sub)
will generate the following SQL:
SELECT "as".* FROM "as" WHERE "as"."x_id" IN (SELECT "xs"."x_y_id" FROM "xs" WHERE "xs"."y" = 'x')
Related
Ok, I feel really stupid for asking this, but it's driving me nuts and I can't figure it out. The docs say I should be able to use select AS in a Rails/ActiveRecord query. So:
d = Dvd.where(id: 1).select("title AS my_title")
Is a valid query and if I do a to_sql on it, it produces the expected SQL:
SELECT title AS my_title FROM `dvd` WHERE `dvd`.`id` = 1
However, d.my_title will give an error:
NoMethodError: undefined method `my_title' for #<ActiveRecord::Relation
I need to be able to use AS since the columns I want to retrieve from different joins have the same name so I can't access them the "regular" way and have to resort to using AS.
I also don't want to resort to using find_by_sql for future compatibility and a possible switch form Mysql to PostGresql.
Just to clarify, what I'm really trying to do is write this SQL in a Railsy way:
SELECT tracks.name AS track_name, artists.name AS artist_name, composers.name AS composer_name, duration
FROM `tracks_cds`
INNER JOIN `tracks` ON `tracks`.`id` = `tracks_cds`.`track_id`
INNER JOIN `artists` ON `artists`.`id` = `tracks_cds`.`artist_id`
INNER JOIN `composers` ON `composers`.`id` = `tracks_cds`.`composer_id`
WHERE cd_id = cd.id
The top example was just a simplification of the fact that SELECT AS will not give you an easy way to refer to custom fields which I find hard to believe.
ActiveRecord automatically creates getter and setter methods for attributes based on the column names in the database, so there will be none defined for my_title.
Regarding the same common names, why not just do this:
d = Dvd.where(id: 1).select("dvds.title")
You can write your sql query and then just pass into ActiveRecord's execute method
query = "SELECT title AS my_title FROM `dvd` WHERE `dvd`.`id` = 1"
result = ActiveRecord::Base.connection.execute(query)
Is there any way to get nested where clauses? e.g.:
SELECT * FROM table WHERE (colA = 'valueA' AND colB = 'valueB') OR (colA = 'valueC' AND colB = 'valueD')
I know I could just write this into a query function call e.g.:
$this->db->query("SELECT ...")
But I was wondering if there was a "proper" way to do it in codeigniter e.g.:
$this->db->where(array('colA'=>'valueA'), array('colB'=>valueB'))->or_where(array('colA'=>'valueC'), array('colB'=>'valueD'))
thanks
With codeigniter 3, now there is, see the update!
There's no where() method usage variant with arrays that would allow you to do that. In these situations i usually just build the part in one long string like this:
$this->db->where("
(
(colA = '".$this->db->escape($v0)."' and colB = '".$this->db->escape($v1)."')
or
(colA = '".$this->db->escape($v2)."' and colB = '".$this->db->escape($v3)."')
)
");
Escaping can be done with escape(does some autodetection) or escape_str or escape_like_str manually depending on what the parameter expected to be or what the predicate in use.
If i'm on a project that uses the Datamapper library, i prefer to use the group_start() and group_end() methods when building these kind of queries, they have a lot of different flavor of these.
Update
Now with, Codeigniter 3 which have grouping methods in the query builder, so you can do ->group_start()s and ->group_end()s.
You can also try like
$this->db->where(condition1);
$this->db->or_where(condition2);
With SQL I can easily do sub-queries like this
User.where(:id => Account.where(..).select(:user_id))
This produces:
SELECT * FROM users WHERE id IN (SELECT user_id FROM accounts WHERE ..)
How can I do this using rails' 3 activerecord/ arel/ meta_where?
I do need/ want real subqueries, no ruby workarounds (using several queries).
Rails now does this by default :)
Message.where(user_id: Profile.select("user_id").where(gender: 'm'))
will produce the following SQL
SELECT "messages".* FROM "messages" WHERE "messages"."user_id" IN (SELECT user_id FROM "profiles" WHERE "profiles"."gender" = 'm')
(the version number that "now" refers to is most likely 3.2)
In ARel, the where() methods can take arrays as arguments that will generate a "WHERE id IN..." query. So what you have written is along the right lines.
For example, the following ARel code:
User.where(:id => Order.where(:user_id => 5)).to_sql
... which is equivalent to:
User.where(:id => [5, 1, 2, 3]).to_sql
... would output the following SQL on a PostgreSQL database:
SELECT "users".* FROM "users" WHERE "users"."id" IN (5, 1, 2, 3)"
Update: in response to comments
Okay, so I misunderstood the question. I believe that you want the sub-query to explicitly list the column names that are to be selected in order to not hit the database with two queries (which is what ActiveRecord does in the simplest case).
You can use project for the select in your sub-select:
accounts = Account.arel_table
User.where(:id => accounts.project(:user_id).where(accounts[:user_id].not_eq(6)))
... which would produce the following SQL:
SELECT "users".* FROM "users" WHERE "users"."id" IN (SELECT user_id FROM "accounts" WHERE "accounts"."user_id" != 6)
I sincerely hope that I have given you what you wanted this time!
I was looking for the answer to this question myself, and I came up with an alternative approach. I just thought I'd share it - hope it helps someone! :)
# 1. Build you subquery with AREL.
subquery = Account.where(...).select(:id)
# 2. Use the AREL object in your query by converting it into a SQL string
query = User.where("users.account_id IN (#{subquery.to_sql})")
Bingo! Bango!
Works with Rails 3.1
Another alternative:
Message.where(user: User.joins(:profile).where(profile: { gender: 'm' })
This is an example of a nested subquery using rails ActiveRecord and using JOINs, where you can add clauses on each query as well as the result :
You can add the nested inner_query and an outer_query scopes in your Model file and use ...
inner_query = Account.inner_query(params)
result = User.outer_query(params).joins("(#{inner_query.to_sql}) alias ON users.id=accounts.id")
.group("alias.grouping_var, alias.grouping_var2 ...")
.order("...")
An example of the scope:
scope :inner_query , -> (ids) {
select("...")
.joins("left join users on users.id = accounts.id")
.where("users.account_id IN (?)", ids)
.group("...")
}
Is this linq query efficient?
var qry = ((from member in this.ObjectContext.TreeMembers.Where(m => m.UserId == userId && m.Birthdate == null)
select member.TreeMemberId).Except(from item in this.ObjectContext.FamilyEvents select item.TreeMemberId));
var mainQry = from mainMember in this.ObjectContext.TreeMembers
where qry.Contains(mainMember.TreeMemberId)
select mainMember;
Will this be translated into multiple sql calls or just one? Can it be optimised? Basically I have 2 tables, I want to select those records from table1 where datetime is null and that record should not exist in table2.
The easiest way to find out if the query will make multiple calls is to set the .Log property of the data context. I typically set it to write to a DebugOutputWriter. A good example for this kind of class can be found here.
For a general way of thinking about it however, if you use a property of your class that does not directly map to a database field in a where clause or a join clause, it will typically make multiple calls. From what you have provided, it looks like this is not the case for your scenario, but I can't absolutely certain and suggest using the method listed above.
I've been able to make this code work using CodeIgniter's db->query as follows:
$sql =
'SELECT mapping_code,zone_name,installation_name
FROM installations,appearances,zones
WHERE
installations.installation_id = zones.installation_fk_id
AND appearances.installation_fk_id = installations.installation_id
AND appearances.zone_fk_id = zones.zone_id
AND
appearances.barcode = ?
';
return $this->db->query($sql,array($barcode));
The 'appearances' table throws a 'not unique table' error if I try
this using the Active Record class.
I need to join appearances on
both the zone and installations tables.
How can I do this?
Take a look at $this->db->join() if you wish to use the ActiveRecord class. Right now you are just using plain SQL which has nothing to do with the ActiveRecord stuff at all.
Does this work when run through a MySQL client like phpMyAdmin, Navicat, etc?