# Code Smell 142 - Queries in Constructors

> TL;DR: Constructors should construct (and probably initialize) objects.

# Problems

- [Coupling](https://maximilianocontieri.com/coupling-the-one-and-only-software-design-problem)

- Side Effects

# Solutions

1. Decouple essential business logic from accidental persistence

2. On persistence classes, run queries in functions other than constructors/destructors

# Context

On legacy code, the database is not correctly separated from business objects.

Constructors should never have side effects.

According to the single responsibility principle, they should only build *valid* objects

# Sample Code

## Wrong

[Gist Url]: # (https://gist.github.com/mcsee/5125008158d94d51b27e8687bbbbf812)
```java
public class Person {
  int childrenCount; 

  public Person(int id) {
    childrenCount = database.sqlCall("SELECT COUNT(CHILDREN) FROM PERSON WHERE ID = " . id); 
  }
}
```

## Right

[Gist Url]: # (https://gist.github.com/mcsee/a84b6d71034c47c1f95590d74bd2126d)
```java
public class Person {
  int childrenCount; 

  // Create a class constructor for the Main class
  public Person(int id, int childrenCount) {
    childrenCount = childrenCount; 
    // We can assign the number in the constructor
    // Accidental Database is decoupled
    // We can test the object
  }
}
```

# Detection

[X] Semi-Automatic 

Our linters can find SQL patterns on constructors and warn us.

# Tags

- Coupling

# Conclusion

Separation of concerns is key and coupling is our main enemy when designing robust software.

# More Info

- [Coupling: The one and only Software Problem](https://maximilianocontieri.com/coupling-the-one-and-only-software-design-problem)

# Credits

<span>Photo by [Callum Hill](https://unsplash.com/@inkyhills) on [Unsplash](https://unsplash.com/s/photos/no)</span>

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> My belief is still, if you get the data structures and their invariants right, most of the code will kind of write itself.

_Peter Deustch_
 
%[https://maximilianocontieri.com/software-engineering-great-quotes]

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This article is part of the CodeSmell Series.

%[https://maximilianocontieri.com/how-to-find-the-stinky-parts-of-your-code]