Rails印象派瑰宝-缺少表“ impressions”的FROM子句条目

Question

I am using Rails 4.2 and have set up the impressionist gem which is working fine for logging page impressions.

I am trying to do exactly the same thing as in this Stackoverflow post but the answer isn't working. I tried:

start_time = 30.days.ago
@mostReadAlbums30Days = Album.joins(:impressions).where("impressions.created_at<='#{Time.now}' and impressions.created_at >= '#{start_time}'").group("impressions.impressionable_id").order("count(impression‌​s.id) DESC")

This produces the following SQL and error message

PG::UndefinedTable: ERROR: missing FROM-clause entry for table "impression‌​s" LINE 1: ... BY impressions.impressionable_id ORDER BY count(impression...

^ : SELECT "albums".* FROM "albums" INNER JOIN "impressions" ON "impressions"."impressionable_id" = "albums"."id" AND "impressions"."impressionable_type" = $1 WHERE (impressions.created_at<='2015-01-02 00:50:18 -0200' and impressions.created_at >= '2014-12-03 02:50:18 UTC') GROUP BY impressions.impressionable_id ORDER BY count(impression‌​s.id) DESC

I'm using Postgresql v9.3.5.2. How can I get this query to work?

Answer 1

The main problem (per comment):

There's a subtle typo in your query. It contains a Unicode Character ZERO WIDTH SPACE (U+200B) between the n and s of ORDER BY count(impression‌s.id) . Look at how impressions gets line-broken in the rendering of the error message, that wouldn't happen otherwise.

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The GROUP BY problem: since the query selects albums.* , it has to GROUP BY albums.id rather than GROUP BY impressions.impressionable_id (assuming that albums.id is a primary key). This is numerically equivalent since the tables are equi-joined through these two fields anyway, but as it seems, the SQL engine in this context can't figure out this equivalency by itself.