imnithin.github.io/multiple-database.html at master · imnithin/imnithin.github.io · GitHub

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    <meta name="keywords" content="multiple databases using rails, multiple databases using ActiveRecord(AR), set tables dynamically in rails">
    <meta name="description" content="one my previous project I dealt with secondary/external database with rails application, which was quite interesting as it was very much dynamic in nature that is I did not have any information available regarding with database">

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                            <h1>Dealing with multiple database in Rails.</h1>
                            <h2 class="subheading">... easy as you see</h2>
                            <!-- <span class="meta">Posted by <a href="#">Start Bootstrap</a> on August 24, 2014</span> -->
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                    <h2 class="section-heading">Multiple Database Connection with ActiveRecord</h2>

                    <p>Off-late I see many questions on StackOverflow on multiple database, also quite a number of approaches doing it wrongly, while I have answered a few, here I'm quite briefing about it.</p>

                    <p>Making a connection when necessary and interacting with secondary database is quite a common approach and poor way of dealing with.</p>

                    <p><h2 class="section-heading">Keeping the configurations at one best place.</h2></p>
database.yml
<pre>
development:
 adapter: [ adapter ]
 database: [DB name]
 username: [your username]
 password: [your password]
 host: [host name]

secondary:
 adapter: [ adapter ]
 database: [ DB name ]
 username: [ username ]
 password: [ password ]
 host: [ host name ]
</pre>

        <p>This keeps the secondary database connection open, which could be a good thing for performance as well allows you to use ActiveRecord of Rails.</p>

<p><h2 class="section-heading">Then comes the connection with AR.</h2></p>

<pre>
Class ExternalDatabaseConnection < ActiveRecord::Base
   establish_connection(:secondary)
   self.table_name = "the_table"
end
</pre>

<p><h2 class="section-heading">Usage</h2></p>

<pre>
ExternalDatabaseConnection.first
ExternalDatabaseConnection.create(.......)
</pre>


<p><h2 class="section-heading">Dynamic way of table setting</h2></p>
<p>In case when table is dynamically set by user. In class you must have to use <p>

<pre>
self.abstract_class = true
attr_protected
</pre>

<p><blockquote>self.abstract_class</blockquote></p>
<p>This tells ActiveRecord that it shouldn’t look for a table to go with that class</p>

<p><blockquote>attr_protected</blockquote></p>
<p>This will permit <b>all</b> attributes related to a table without defining it manually, in Rails 4, there must be different way.</p>

<p><blockquote>ExternalDatabaseConnection.table_name = "table1"</blockquote></p>
<p><b>ExternalDatabaseConnection</b> is ready to serve as in AR object, with the related table set.</p>

<p><h2 class="section-heading">The Problem with dynamic way.</h2></p>

<p>Dynamic way of changing table name, Consider..</p>

<p><pre>
ExternalDatabaseConnection.table_name = "users"
ExternalDatabaseConnection.column_names => ["name", "location", "email", "verified"]

ExternalDatabaseConnection.table_name = "posts"
ExternalDatabaseConnection.column_names => ["name", "location", "email", "verified"]
</pre></p>

<p>You see the problem there, its shows the attributes of initially set table, hence you no longer hold the instance of new object.</p>

<p><h2 class="section-heading">The Fix.</h2></p>

<pre>
ExternalDatabaseConnection.reset_column_information
</pre>

<p>To override the previous object - table information, reset_column_information method on the class object must be called before setting new table.</p>


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