ruby/sql syntax undefined method `query' for nil:NilClass (NoMethodError)

Question

I keep getting the following error message:

.rb:215:in block in collectData': undefined methodquery' for nil:NilClass (NoMethodError)

This is my code:

databases = {
  'aaa': Mysql2::Client.new(
      host:     '',
      username: '',
      password: '',
      database: ''
    ),
  'bbb': Mysql2::Client.new(
      host:     '',
      username: '',
      password: '',
      database: ''
    )
}

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = databases[row['db']]
    row2 = next_db.query("SELECT...")

I take the name of the db from my first sql query, client1.query, for example row['db'] = "aaa" or "bbb".

For the test something like this also not working:

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = databases['aaa']
    row2 = next_db.query("SELECT...")

And something like this works:

client1 = Mysql2::Client.new(
    host:     '',
    username: '',
    password: '',
    database: ''
  )

client2 = Mysql2::Client.new(
    host:     '',
    username: '',
    password: '',
    database: ''
  )

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = client2
    row2 = next_db.query("SELECT ")

I can't seem to understand why my method is "undefined".

Please help,

Thanks !

br3nt · Accepted Answer · 2016-07-09 09:07:23Z

2

So you have your hash of databases, and you want to query the next database based off the result from the first database query...

Basically the code you have is right except you never check for nil. In this case, whatever is being returned by row['db'], a matching database is not found in the databases hash. I'm going to assume that this is an exception that needs to be fixed.

So...

# hash of databases
databases = {
   'aaa' => Mysql2::Client.new(host: '', username: '', password: '', database: ''),
   'bbb' => Mysql2::Client.new(host: '',username: '', password: '', database: '')
 }

# get the first database
client1 = databases['aaa'] 

# perform the query
client1.query("bla bla bla").each do |row|
  if row['mmm'] == 99
    # look in next db
    client2 = databases[row['db']]

    if (client2)
      row2 = client2.query("SELECT...")
    else
      raise "Database `#{row['db']}` is not defined."
    end
  end
end

edited Jul 9, 2016 at 9:07

answered Jul 9, 2016 at 2:52

br3nt

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3 Comments

user6558246 Over a year ago

I still got the same error "Database aaa is not defined", I fixed that with the answer above, thank you.

br3nt Over a year ago

@AlexBrickman oops, was using the wrong syntax for the hash... Fixed now

br3nt Over a year ago

The colon was converting the keys to symbols, therefore databases[row['db']] was returning nil as it was finding trying to find the db by a string, not a symbol.

Berlin · Accepted Answer · 2016-07-09 07:50:18Z

0

try to change the hash of databases to:

databases = {
   'aaa' => Mysql2::Client.new(host: '', username: '', password: '', database: ''),
   'bbb' => Mysql2::Client.new(host: '',username: '', password: '', database: '')
 }

answered Jul 9, 2016 at 7:50

Berlin

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2 Comments

user6558246 Over a year ago

its works ! but why with => and not with : , is someone can explain?

Berlin Over a year ago

please see @br3nt explanation.

Nic Nilov · Accepted Answer · 2016-07-08 21:59:27Z

0

Your client instances reside not in the client variable like in your working example but within a hash. The reason for the error is that you're trying to call query method on a variable that has nil value.

You should access your clients appropriately:

databases['aaa'].query("
  ").each do |row|
            if row['mmm'] == 99
              # Look in next db
                next_db = client
                row2 = next_db.query("
                SELECT " )

Of course to achieve the logic you're aiming for, it will be necessary to walk the databases hash as you do the steps.

edited Jul 8, 2016 at 21:59

answered Jul 8, 2016 at 21:56

Nic Nilov

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4 Comments

user6558246 Over a year ago

thanks, but I need to access the db by the name that I get from my first sql query, row['db'] can be aaa or bbb.

user6558246 Over a year ago

i want to access to aaa and bbb by the name that i get from my first query.

Nic Nilov Over a year ago

Then write a recursive function, with each step using appropriate client of the ones contained in the databases. This aspect is not related to your original question which has received an answer.

user6558246 Over a year ago

I asked here to get some help how should I do it, because even if I do something like this "next_db = databases['aaa']" it doesn't work.

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