ref() function for model dependencies in dbt - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of ref() in a dbt model SQL

Given the following dbt model SQL code, what will be the fully qualified table name generated by the ref('customers') function if the project is deployed in the analytics schema of the prod database?

dbt

select * from {{ ref('customers') }}

A "prod"."analytics"."customers"

B "analytics"."prod"."customers"

C "prod"."customers"."analytics"

D "customers"."analytics"."prod"

💡 Hint

Remember that ref() resolves to the database, schema, and table name in that order.

The ref() function in dbt returns the fully qualified name of the referenced model in the format database.schema.table. Since the project is deployed in the prod database and analytics schema, the correct output is "prod"."analytics"."customers".

❓ data_output

intermediate

Number of rows returned using ref() in a join

Consider two dbt models: orders and customers. Consider the following model SQL:

select o.order_id, c.customer_name
from {{ ref('orders') }} o
join {{ ref('customers') }} c on o.customer_id = c.customer_id

If orders has 100 rows and customers has 10 rows, and every order has a matching customer, how many rows will this query return?

A 110

B 10

C 1000

D 100

💡 Hint

Think about how a join works when every order has a matching customer.

The join matches each order to exactly one customer. Since there are 100 orders and each has a matching customer, the result will have 100 rows.

🔧 Debug

advanced

Identify the error caused by incorrect use of ref()

What error will occur when running this dbt model SQL?

select * from {{ ref(customers) }}

Note: customers is not in quotes.

A Runtime error: table not found

B Compilation error: 'customers' is not defined

C Syntax error: missing quotes around string

D No error, runs successfully

💡 Hint

Look at how the argument to ref() is passed.

The ref() function requires a string argument. Without quotes, customers is treated as a variable which is not defined, causing a compilation error.

🧠 Conceptual

advanced

Understanding model dependency graph with ref()

In dbt, if model A uses {{ ref('B') }} and model B uses {{ ref('C') }}, which models will be built before A?

A Only model <code>B</code> will be built before <code>A</code>

B Only model <code>C</code> will be built before <code>A</code>

C Models <code>B</code> and <code>C</code> will be built before <code>A</code>

D Models <code>A</code>, <code>B</code>, and <code>C</code> are built simultaneously

💡 Hint

Think about how dependencies chain in dbt models.

dbt builds models in dependency order. Since A depends on B, and B depends on C, both B and C must be built before A.

🚀 Application

expert

Predict the effect of changing ref() argument on model build order

You have three models: sales, customers, and regions. The sales model uses {{ ref('customers') }}. You change the sales model to use {{ ref('regions') }} instead. What is the impact on the build order?

A The <code>sales</code> model will now build after <code>regions</code> instead of <code>customers</code>

B The build order remains the same; <code>sales</code> builds after both <code>customers</code> and <code>regions</code>

C The <code>sales</code> model will build before both <code>customers</code> and <code>regions</code>

D The <code>sales</code> model will build independently without waiting for any other model

💡 Hint

Consider how changing the ref() target changes dependencies.

Changing the ref() argument changes the dependency. Now sales depends on regions instead of customers, so it builds after regions.

Practice

1. What is the main purpose of the ref() function in dbt?

easy

A. To create new database users

B. To write raw SQL queries inside dbt models

C. To link models and define dependencies between them

D. To schedule dbt runs automatically

Solution

1. Step 1: Understand the role of ref()

   The ref() function is used to link one model to another in dbt, so dbt knows the order to run models and their dependencies.

2. Step 2: Identify what ref() does not do

   It does not write raw SQL, create users, or schedule runs. Its main role is linking models.

3. Final Answer:

   To link models and define dependencies between them -> Option C

4. Quick Check:

   ref() links models = A [OK]

Hint: Remember: ref() connects models, not SQL or users [OK]

Common Mistakes:

• Thinking ref() writes SQL code
• Confusing ref() with scheduling tools
• Assuming ref() manages database users

2. Which of the following is the correct syntax to reference a model named customers inside another model using ref()?

easy

A. select * from {{ ref('customers') }}

B. select * from ref('customers')

C. select * from ref(customers)

D. select * from {{ ref(customers) }}

Solution

1. Step 1: Recall dbt Jinja syntax for ref()

   In dbt, ref() must be wrapped in double curly braces and the model name must be a string in quotes.

2. Step 2: Check each option

   select * from {{ ref('customers') }} uses {{ ref('customers') }} which is correct. Options B and C miss the curly braces or quotes. select * from {{ ref(customers) }} misses quotes around the model name.

3. Final Answer:

   select * from {{ ref('customers') }} -> Option A

4. Quick Check:

   Use {{ ref('model_name') }} syntax = A [OK]

Hint: Always use {{ ref('model_name') }} with quotes and braces [OK]

Common Mistakes:

• Omitting curly braces {{ }}
• Not putting model name in quotes
• Using ref() without Jinja syntax

3. Given the following dbt model code, what will be the output SQL after compilation if the orders model exists?

select order_id, customer_id
from {{ ref('orders') }}

medium

A. select order_id, customer_id from orders

B. select order_id, customer_id from {{ ref('orders') }}

C. select order_id, customer_id from dbt.orders

D. select order_id, customer_id from ref('orders')

Solution

1. Step 1: Understand what ref() compiles to

   The ref() function compiles to the actual table name of the referenced model, usually just the model name like 'orders'.

2. Step 2: Check the compiled SQL output

   The compiled SQL replaces {{ ref('orders') }} with orders, so the output is select order_id, customer_id from orders.

3. Final Answer:

   select order_id, customer_id from orders -> Option A

4. Quick Check:

   ref('orders') compiles to orders = C [OK]

Hint: ref() compiles to the model's table name without braces [OK]

Common Mistakes:

• Leaving ref() uncompiled in SQL
• Adding extra schema prefix without config
• Using ref() as a string literal

4. You wrote this dbt model code:

select * from ref('sales')

When you run dbt, you get an error. What is the problem?

medium

A. Quotes around 'sales' should be removed

B. Model name 'sales' does not exist

C. ref() cannot be used inside select statements

D. Missing double curly braces around ref()

Solution

1. Step 1: Check the syntax of ref() usage

   In dbt, ref() must be wrapped in double curly braces to be interpreted as Jinja code.

2. Step 2: Identify the error cause

   The code uses ref('sales') without {{ }}, so dbt treats it as plain text, causing an error.

3. Final Answer:

   Missing double curly braces around ref() -> Option D

4. Quick Check:

   Use {{ ref('model') }} not ref('model') alone = D [OK]

Hint: Always wrap ref() in {{ }} to avoid errors [OK]

Common Mistakes:

• Forgetting {{ }} around ref()
• Assuming ref() works without Jinja
• Removing quotes from model name

5. You have two models: customers and orders. You want to create a new model customer_orders that joins these two. Which is the best way to use ref() to ensure correct dependencies and flexible naming?

hard

A. select c.customer_id, o.order_id from customers c join orders o on c.customer_id = o.customer_id

B. select c.customer_id, o.order_id from {{ ref('customers') }} c join {{ ref('orders') }} o on c.customer_id = o.customer_id

C. select c.customer_id, o.order_id from 'customers' c join 'orders' o on c.customer_id = o.customer_id

D. select c.customer_id, o.order_id from ref('customers') c join ref('orders') o on c.customer_id = o.customer_id

Solution

1. Step 1: Use ref() with correct Jinja syntax for both models

   To link models and ensure dbt knows dependencies, use {{ ref('model_name') }} for both customers and orders.

2. Step 2: Avoid hardcoding table names or missing Jinja syntax

   Options A and C hardcode names or use quotes incorrectly. select c.customer_id, o.order_id from ref('customers') c join ref('orders') o on c.customer_id = o.customer_id misses curly braces, so it won't compile.

3. Final Answer:

   select c.customer_id, o.order_id from {{ ref('customers') }} c join {{ ref('orders') }} o on c.customer_id = o.customer_id -> Option B

4. Quick Check:

   Use {{ ref('model') }} for all dependencies = B [OK]

Hint: Use {{ ref('model') }} for all model references [OK]

Common Mistakes:

• Hardcoding table names instead of using ref()
• Forgetting curly braces around ref()
• Using quotes incorrectly around model names